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.

AK/RefPtr.h

@@ -143,7 +143,7 @@ public:
         Adopt
     };
 
-    RefPtr() { }
+    RefPtr() = default;
     RefPtr(const T* ptr)
         : m_bits(PtrTraits::as_bits(const_cast<T*>(ptr)))
     {
@@ -205,7 +205,6 @@ public:
             m_bits.store(0xe0e0e0e0, AK::MemoryOrder::memory_order_relaxed);
 #endif
     }
-    RefPtr(std::nullptr_t) { }
 
     template<typename U>
     RefPtr(const OwnPtr<U>&) = delete;