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.

Services/ImageDecoder/ClientConnection.cpp

@@ -65,14 +65,14 @@ OwnPtr<Messages::ImageDecoderServer::DecodeImageResponse> ClientConnection::hand
 #ifdef IMAGE_DECODER_DEBUG
         dbgln("Could not map encoded data buffer");
 #endif
-        return nullptr;
+        return {};
     }
 
     if (message.encoded_size() > (size_t)encoded_buffer->size()) {
 #ifdef IMAGE_DECODER_DEBUG
         dbgln("Encoded buffer is smaller than encoded size");
 #endif
-        return nullptr;
+        return {};
     }
 
 #ifdef IMAGE_DECODER_DEBUG