Determine if HttpJson should allow null response back

[lqiu96]

Check if this is the correct flow for GAX HttpJson: https://github.com/googleapis/gapic-generator-java/blob/5863737238b302c6befd9670f83896be1cf190cb/gax-java/gax-httpjson/src/main/java/com/google/api/gax/httpjson/HttpJsonClientCallImpl.java#L345-L346

Currently, a null response will treated as a server error. This may be because we are REST over gRPC and gRPC requires a response in the payload.

The REST world does allow for a null response: https://stackoverflow.com/questions/46242200/post-with-no-response-body-which-response-code-is-better-201-or-204#:~:text=or%20deleting%20%2D%20204.-,201%20Created,by%20the%20effective%20request%20URI

We should investigate and determine if this is intended/ how we want to keep it.