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;