Fix return type from intercept_service

[mahlberg]

Description of change

Updating return type for ServerInterceptor.intercept_service, according to the gRPC documentation this can also return None. This is the documentation for the return type:

An RpcMethodHandler with which the RPC may be serviced if the interceptor chooses to service this RPC, or None otherwise.

Minimum Reproducible Example

This is a one-line change and I believe the provided typing test is sufficient.

Checklist:

• I have verified my MRE is sufficient to demonstrate the issue and solution by attempting to execute it myself
  • OR I believe my contribution is minor enough that a typing test is sufficient.
• I have added tests to typesafety/test_grpc.yml for all APIs added or changed by this PR
• I have removed any code generation notices from anything seeded using mypy-protobuf.

[shabbyrobe]

Thank you!