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/Format.h

@@ -266,7 +266,7 @@ struct StandardFormatter {
 
 template<typename T>
 struct Formatter<T, typename EnableIf<IsIntegral<T>::value>::Type> : StandardFormatter {
-    Formatter() { }
+    Formatter() = default;
     explicit Formatter(StandardFormatter formatter)
         : StandardFormatter(formatter)
     {
@@ -277,7 +277,7 @@ struct Formatter<T, typename EnableIf<IsIntegral<T>::value>::Type> : StandardFor
 
 template<>
 struct Formatter<StringView> : StandardFormatter {
-    Formatter() { }
+    Formatter() = default;
     explicit Formatter(StandardFormatter formatter)
         : StandardFormatter(formatter)
     {
@@ -338,7 +338,7 @@ struct Formatter<float> : StandardFormatter {
 };
 template<>
 struct Formatter<double> : StandardFormatter {
-    Formatter() { }
+    Formatter() = default;
     explicit Formatter(StandardFormatter formatter)
         : StandardFormatter(formatter)
     {