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/Span.h

@@ -105,10 +105,7 @@ class Span : public Detail::Span<T> {
 public:
     using Detail::Span<T>::Span;
 
-    ALWAYS_INLINE constexpr Span(std::nullptr_t)
-        : Span()
-    {
-    }
+    constexpr Span() = default;
 
     ALWAYS_INLINE constexpr Span(const Span& other)
         : Span(other.m_values, other.m_size)