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/Project.cpp

@@ -43,7 +43,7 @@ Project::~Project()
 OwnPtr<Project> Project::open_with_root_path(const String& root_path)
 {
     if (!Core::File::is_directory(root_path))
-        return nullptr;
+        return {};
     return adopt_own(*new Project(root_path));
 }