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.cpp

@@ -213,7 +213,7 @@ Vector<StringView> String::split_view(const char separator, bool keep_empty) con
 ByteBuffer String::to_byte_buffer() const
 {
     if (!m_impl)
-        return nullptr;
+        return {};
     return ByteBuffer::copy(reinterpret_cast<const u8*>(characters()), length());
 }