Fix _stream_unary not working with empty iterables (#422)

* Fixed `do_many_things` method in `ThingService`
* Added test for empty iterables
* Fixed `_stream_unary` not sending request first

tests/grpc/thing_service.py

@@ -27,7 +27,7 @@ class ThingService:
     async def do_many_things(
         self, stream: "grpclib.server.Stream[DoThingRequest, DoThingResponse]"
     ):
-        thing_names = [request.name for request in stream]
+        thing_names = [request.name async for request in stream]
         if self.test_hook is not None:
             self.test_hook(stream)
         await stream.send_message(DoThingResponse(thing_names))