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/Userspace.h

@@ -44,7 +44,7 @@ template<typename T, typename EnableIf<IsPointer<T>::value, int>::Type = 0>
 
 class Userspace {
 public:
-    Userspace() { }
+    Userspace() = default;
 
     operator bool() const { return m_ptr; }
     operator FlatPtr() const { return (FlatPtr)m_ptr; }