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/String.h

@@ -58,9 +58,9 @@ namespace AK {
 
 class String {
 public:
-    ~String() { }
+    ~String() = default;
 
-    String() { }
+    String() = default;
     String(const StringView&);
 
     String(const String& other)
@@ -153,7 +153,13 @@ public:
 
     [[nodiscard]] bool copy_characters_to_buffer(char* buffer, size_t buffer_size) const;
 
-    ALWAYS_INLINE ReadonlyBytes bytes() const { return m_impl ? m_impl->bytes() : nullptr; }
+    ALWAYS_INLINE ReadonlyBytes bytes() const
+    {
+        if (m_impl) {
+            return m_impl->bytes();
+        }
+        return {};
+    }
 
     ALWAYS_INLINE const char& operator[](size_t i) const
     {