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/Lock.cpp

@@ -123,7 +123,7 @@ void Lock::lock(Mode mode)
                     ASSERT_NOT_REACHED();
                 }
                 m_lock.store(false, AK::memory_order_release);
-            } while (m_queue.wait_on(nullptr, m_name) == Thread::BlockResult::NotBlocked);
+            } while (m_queue.wait_on({}, m_name) == Thread::BlockResult::NotBlocked);
         } else {
             // I don't know *who* is using "m_lock", so just yield.
             Scheduler::yield_from_critical();