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/LibLine/InternalFunctions.cpp

@@ -43,7 +43,7 @@ Function<bool(Editor&)> Editor::find_internal_function(const StringView& name)
 
     ENUMERATE_EDITOR_INTERNAL_FUNCTIONS(__ENUMERATE)
 
-    return nullptr;
+    return {};
 }
 
 void Editor::search_forwards()