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/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;