Fix JSON printing of Any of an empty message in C++Proto.

This check was likely based on confusion about presence: if the 'bytes' field of Any supported presence and was unset it would be a malformed message.

However, as a Proto3 message the bytes field is implicit presence, and an empty message serializes to 0-bytes. It is perfectly valid to pack an empty message into an Any, and there's no distinguishing between "they forgot to set the value" and "actual zero-length serialization".

This wasn't caught by a user sooner since it is already allowed when allow_legacy_nonconformant_behavior is on, and that setting is on by default and rarely disabled.

https://github.com/protocolbuffers/protobuf/issues/24937

PiperOrigin-RevId: 848253357