AK: Simplify constructors and conversions from nullptr_t

Problem:
- Many constructors are defined as `{}` rather than using the ` =
  default` compiler-provided constructor.
- Some types provide an implicit conversion operator from `nullptr_t`
  instead of requiring the caller to default construct. This violates
  the C++ Core Guidelines suggestion to declare single-argument
  constructors explicit
  (https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#c46-by-default-declare-single-argument-constructors-explicit).

Solution:
- Change default constructors to use the compiler-provided default
  constructor.
- Remove implicit conversion operators from `nullptr_t` and change
  usage to enforce type consistency without conversion.

Libraries/LibMarkdown/CodeBlock.cpp

@@ -112,13 +112,13 @@ String CodeBlock::render_for_terminal(size_t) const
 OwnPtr<CodeBlock> CodeBlock::parse(Vector<StringView>::ConstIterator& lines)
 {
     if (lines.is_end())
-        return nullptr;
+        return {};
 
     constexpr auto tick_tick_tick = "```";
 
     StringView line = *lines;
     if (!line.starts_with(tick_tick_tick))
-        return nullptr;
+        return {};
 
     // Our Markdown extension: we allow
     // specifying a style and a language
@@ -134,7 +134,7 @@ OwnPtr<CodeBlock> CodeBlock::parse(Vector<StringView>::ConstIterator& lines)
     StringView style_spec = line.substring_view(3, line.length() - 3);
     auto spec = Text::parse(style_spec);
     if (!spec.has_value())
-        return nullptr;
+        return {};
 
     ++lines;
 

Libraries/LibMarkdown/Document.cpp

@@ -119,7 +119,7 @@ OwnPtr<Document> Document::parse(const StringView& str)
 
         auto line = Paragraph::Line::parse(lines);
         if (!line)
-            return nullptr;
+            return {};
 
         paragraph_lines.append(line.release_nonnull());
     }

Libraries/LibMarkdown/Heading.cpp

@@ -62,7 +62,7 @@ String Heading::render_for_terminal(size_t) const
 OwnPtr<Heading> Heading::parse(Vector<StringView>::ConstIterator& lines)
 {
     if (lines.is_end())
-        return nullptr;
+        return {};
 
     const StringView& line = *lines;
     size_t level;
@@ -73,12 +73,12 @@ OwnPtr<Heading> Heading::parse(Vector<StringView>::ConstIterator& lines)
     }
 
     if (!level || level >= line.length() || line[level] != ' ')
-        return nullptr;
+        return {};
 
     StringView title_view = line.substring_view(level + 1, line.length() - level - 1);
     auto text = Text::parse(title_view);
     if (!text.has_value())
-        return nullptr;
+        return {};
 
     auto heading = make<Heading>(move(text.value()), level);
 

Libraries/LibMarkdown/HorizontalRule.cpp

@@ -47,19 +47,19 @@ String HorizontalRule::render_for_terminal(size_t view_width) const
 OwnPtr<HorizontalRule> HorizontalRule::parse(Vector<StringView>::ConstIterator& lines)
 {
     if (lines.is_end())
-        return nullptr;
+        return {};
 
     const StringView& line = *lines;
 
     if (line.length() < 3)
-        return nullptr;
+        return {};
     if (!line.starts_with('-') && !line.starts_with('_') && !line.starts_with('*'))
-        return nullptr;
+        return {};
 
     auto first_character = line.characters_without_null_termination()[0];
     for (auto ch : line) {
         if (ch != first_character)
-            return nullptr;
+            return {};
     }
 
     ++lines;

Libraries/LibMarkdown/List.cpp

@@ -122,20 +122,20 @@ OwnPtr<List> List::parse(Vector<StringView>::ConstIterator& lines)
             if (first)
                 is_ordered = appears_ordered;
             else if (is_ordered != appears_ordered)
-                return nullptr;
+                return {};
 
             if (!flush_item_if_needed())
-                return nullptr;
+                return {};
 
             while (offset + 1 < line.length() && line[offset + 1] == ' ')
                 offset++;
 
         } else {
             if (first)
-                return nullptr;
+                return {};
             for (size_t i = 0; i < offset; i++) {
                 if (line[i] != ' ')
-                    return nullptr;
+                    return {};
             }
         }
 
@@ -149,7 +149,7 @@ OwnPtr<List> List::parse(Vector<StringView>::ConstIterator& lines)
     }
 
     if (!flush_item_if_needed() || first)
-        return nullptr;
+        return {};
     return make<List>(move(items), is_ordered);
 }
 

Libraries/LibMarkdown/Paragraph.cpp

@@ -61,11 +61,11 @@ String Paragraph::render_for_terminal(size_t) const
 OwnPtr<Paragraph::Line> Paragraph::Line::parse(Vector<StringView>::ConstIterator& lines)
 {
     if (lines.is_end())
-        return nullptr;
+        return {};
 
     auto text = Text::parse(*lines++);
     if (!text.has_value())
-        return nullptr;
+        return {};
 
     return make<Paragraph::Line>(text.release_value());
 }

Libraries/LibMarkdown/Table.cpp

@@ -118,12 +118,12 @@ OwnPtr<Table> Table::parse(Vector<StringView>::ConstIterator& lines)
     auto peek_it = lines;
     auto first_line = *peek_it;
     if (!first_line.starts_with('|'))
-        return nullptr;
+        return {};
 
     ++peek_it;
 
     if (peek_it.is_end())
-        return nullptr;
+        return {};
 
     auto header_segments = first_line.split_view('|', true);
     auto header_delimiters = peek_it->split_view('|', true);
@@ -141,10 +141,10 @@ OwnPtr<Table> Table::parse(Vector<StringView>::ConstIterator& lines)
     ++peek_it;
 
     if (header_delimiters.size() != header_segments.size())
-        return nullptr;
+        return {};
 
     if (header_delimiters.is_empty())
-        return nullptr;
+        return {};
 
     size_t total_width = 0;
 
@@ -154,7 +154,7 @@ OwnPtr<Table> Table::parse(Vector<StringView>::ConstIterator& lines)
     for (size_t i = 0; i < header_segments.size(); ++i) {
         auto text_option = Text::parse(header_segments[i]);
         if (!text_option.has_value())
-            return nullptr; // An invalid 'text' in the header should just fail the table parse.
+            return {}; // An invalid 'text' in the header should just fail the table parse.
 
         auto text = text_option.release_value();
         auto& column = table->m_columns[i];