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/OwnPtr.h

@@ -34,7 +34,7 @@ namespace AK {
 template<typename T>
 class OwnPtr {
 public:
-    OwnPtr() { }
+    OwnPtr() = default;
     explicit OwnPtr(T* ptr)
         : m_ptr(ptr)
     {
@@ -57,7 +57,6 @@ public:
         : m_ptr(other.leak_ptr())
     {
     }
-    OwnPtr(std::nullptr_t) {};
     ~OwnPtr()
     {
         clear();