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.

Applications/TextEditor/TextEditorWidget.cpp

@@ -445,7 +445,7 @@ TextEditorWidget::TextEditorWidget()
 
     auto& syntax_menu = view_menu.add_submenu("Syntax");
     m_plain_text_highlight = GUI::Action::create_checkable("Plain text", [&](auto&) {
-        m_editor->set_syntax_highlighter(nullptr);
+        m_editor->set_syntax_highlighter({});
         m_editor->update();
     });
     m_plain_text_highlight->set_checked(true);