Fix stream/sink crash when RocketException doesn't have data
Summary:
During RocketClient shutdown, we fail all sent requests and active stream/sinks with the `error_` stored in `RocketClient`: https://github.com/facebook/fbthrift/blob/main/thrift/lib/cpp2/transport/rocket/client/RocketClient.cpp#L1260-L1281
However, this `error_` could be a `RocketException` with an empty data buffer which causes a crash when propagated to active streams/sinks because that code path assumes that there is always data in the `RocketException`.
Reviewed By: maalbash
Differential Revision:
D43365746
fbshipit-source-id:
2e0a9cb1aa13862a4dfa9d5ee7ab37b497e57a1c