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.

Kernel/Syscalls/thread.cpp

@@ -132,7 +132,7 @@ int Process::sys$join_thread(pid_t tid, Userspace<void**> exit_value)
     // NOTE: pthread_join() cannot be interrupted by signals. Only by death.
     for (;;) {
         KResult try_join_result(KSuccess);
-        auto result = current_thread->block<Thread::JoinBlocker>(nullptr, *thread, try_join_result, joinee_exit_value);
+        auto result = current_thread->block<Thread::JoinBlocker>({}, *thread, try_join_result, joinee_exit_value);
         if (result == Thread::BlockResult::NotBlocked) {
             if (try_join_result.is_error())
                 return try_join_result.error();