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/write.cpp

@@ -97,7 +97,7 @@ ssize_t Process::do_write(FileDescription& description, const UserOrKernelBuffer
                 return total_nwritten;
             }
             auto unblock_flags = Thread::FileBlocker::BlockFlags::None;
-            if (Thread::current()->block<Thread::WriteBlocker>(nullptr, description, unblock_flags).was_interrupted()) {
+            if (Thread::current()->block<Thread::WriteBlocker>({}, description, unblock_flags).was_interrupted()) {
                 if (total_nwritten == 0)
                     return -EINTR;
             }