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/ProtocolServer/HttpProtocol.cpp

@@ -53,7 +53,7 @@ OwnPtr<Download> HttpProtocol::start_download(ClientConnection& client, const St
 
     auto pipe_result = get_pipe_for_download();
     if (pipe_result.is_error())
-        return nullptr;
+        return {};
 
     auto output_stream = make<OutputFileStream>(pipe_result.value().write_fd);
     output_stream->make_unbuffered();