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

src/betterproto/grpc/grpclib_client.py

@@ -127,6 +127,7 @@ class ServiceStub(ABC):
             response_type,
             **self.__resolve_request_kwargs(timeout, deadline, metadata),
         ) as stream:
+            await stream.send_request()
             await self._send_messages(stream, request_iterator)
             response = await stream.recv_message()
         assert response is not None

tests/grpc/test_grpclib_client.py

@@ -272,3 +272,27 @@ async def test_async_gen_for_stream_stream_request():
         assert response_index == len(
             expected_things
         ), "Didn't receive all expected responses"
+
+
+@pytest.mark.asyncio
+async def test_stream_unary_with_empty_iterable():
+    things = []  # empty
+
+    async with ChannelFor([ThingService()]) as channel:
+        client = ThingServiceClient(channel)
+        requests = [DoThingRequest(name) for name in things]
+        response = await client.do_many_things(requests)
+        assert len(response.names) == 0
+
+
+@pytest.mark.asyncio
+async def test_stream_stream_with_empty_iterable():
+    things = []  # empty
+
+    async with ChannelFor([ThingService()]) as channel:
+        client = ThingServiceClient(channel)
+        requests = [GetThingRequest(name) for name in things]
+        responses = [
+            response async for response in client.get_different_things(requests)
+        ]
+        assert len(responses) == 0

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))