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/Utf32View.h

@@ -38,8 +38,8 @@ class Utf32CodepointIterator {
     friend class Utf32View;
 
 public:
-    Utf32CodepointIterator() { }
-    ~Utf32CodepointIterator() { }
+    Utf32CodepointIterator() = default;
+    ~Utf32CodepointIterator() = default;
 
     bool operator==(const Utf32CodepointIterator& other) const
     {
@@ -83,7 +83,7 @@ class Utf32View {
 public:
     using Iterator = Utf32CodepointIterator;
 
-    Utf32View() { }
+    Utf32View() = default;
     Utf32View(const u32* code_points, size_t length)
         : m_code_points(code_points)
         , m_length(length)