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/Net/TCPSocket.cpp

@@ -400,7 +400,7 @@ KResult TCPSocket::protocol_connect(FileDescription& description, ShouldBlock sh
     if (should_block == ShouldBlock::Yes) {
         locker.unlock();
         auto unblock_flags = Thread::FileBlocker::BlockFlags::None;
-        if (Thread::current()->block<Thread::ConnectBlocker>(nullptr, description, unblock_flags).was_interrupted())
+        if (Thread::current()->block<Thread::ConnectBlocker>({}, description, unblock_flags).was_interrupted())
             return KResult(-EINTR);
         locker.lock();
         ASSERT(setup_state() == SetupState::Completed);