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.

DevTools/HackStudio/Editor.cpp

@@ -436,7 +436,7 @@ void Editor::set_document(GUI::TextDocument& doc)
         m_language_client = get_language_client<LanguageClients::Shell::ServerConnection>(project().root_path());
         break;
     default:
-        set_syntax_highlighter(nullptr);
+        set_syntax_highlighter({});
     }
 
     if (m_language_client) {