idl-compiler: avoid unused variable in empty struct deserialisers
idl-compiler: avoid unused variable in empty struct deserialisers

Deserialisers generated by IDL compiler first create a substream
covering the deserialised structure and then skip and read appropriate
members. If there are no members the substream will be unused and prompt
the compiler to emit a warning.

scylladb/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla