Fix dialyzer -Werror_handling issues for introspection
Fix dialyzer -Werror_handling issues for introspection

Fix issues in the generated introspection functions for the
-Werror_handling dialyzer flags, when the .proto file contains no
messages, no enums or no rpc definitions.

The fix consistes of generating "-spec fn(...) -> no_return()."

mmmries/gpb