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/FileSystem/FIFO.cpp

@@ -75,7 +75,7 @@ KResultOr<NonnullRefPtr<FileDescription>> FIFO::open_direction_blocking(FIFO::Di
 
         if (m_writers == 0) {
             locker.unlock();
-            m_write_open_queue.wait_on(nullptr, "FIFO");
+            m_write_open_queue.wait_on({}, "FIFO");
             locker.lock();
         }
     }
@@ -85,7 +85,7 @@ KResultOr<NonnullRefPtr<FileDescription>> FIFO::open_direction_blocking(FIFO::Di
 
         if (m_readers == 0) {
             locker.unlock();
-            m_read_open_queue.wait_on(nullptr, "FIFO");
+            m_read_open_queue.wait_on({}, "FIFO");
             locker.lock();
         }
     }