PubsubIO returns empty message bodies for all messages read

> Both the JSON and gRPC implementation return empty message bodies for all messages read (using readMessages). When running with the dataflow-specific reader, this doesn't happen and the message bodies have the content as expected. I took a pipeline that works as expected on dataflow using PubsubIO.Read, added the experiment flag, and then my pipeline broke from empty message bodies. This obviously blocked me from really experimenting much more.

Imported from Jira [BEAM-7716](https://issues.apache.org/jira/browse/BEAM-7716). Original Jira may contain additional context.
Reported by: SteveNiemitz.