Set serialized_on_wire when message contains only lists

This fixes a bug where serialized_on_wire was not set when a message contained only repeated values (eg in a list or map). The fix here is to just set it to true in the `parse` method as soon as we receive any valid data. This also adds a test to expose the behavior.

betterproto/__init__.py

@@ -747,6 +747,9 @@ class Message(ABC):
                 self._unknown_fields += parsed.raw
                 continue
 
+            # Got some data over the wire
+            self._serialized_on_wire = True
+
             meta = self._betterproto.meta_by_field_name[field_name]
 
             value: Any

betterproto/tests/inputs/repeated/repeated.proto

@@ -1,5 +1,11 @@
 syntax = "proto3";
 
+package repeated;
+
 message Test {
     repeated string names = 1;
 }
+
+service ExampleService {
+  rpc DoThing (Test) returns (Test);
+}

betterproto/tests/inputs/repeated/test_repeated.py

@@ -0,0 +1,39 @@
+from typing import Dict
+
+import grpclib.const
+import grpclib.server
+import pytest
+from grpclib.testing import ChannelFor
+
+import betterproto
+from betterproto.tests.output_betterproto.repeated.repeated import (
+    ExampleServiceStub,
+    Test,
+)
+
+
+class ExampleService:
+    async def DoThing(
+        self, stream: "grpclib.server.Stream[Test, Test]"
+    ):
+        request = await stream.recv_message()
+        await stream.send_message(request)
+
+    def __mapping__(self) -> Dict[str, grpclib.const.Handler]:
+        return {
+            "/repeated.ExampleService/DoThing": grpclib.const.Handler(
+                self.DoThing,
+                grpclib.const.Cardinality.UNARY_UNARY,
+                Test,
+                Test,
+            ),
+        }
+
+
+@pytest.mark.asyncio
+async def test_sets_serialized_on_wire() -> None:
+    async with ChannelFor([ExampleService()]) as channel:
+        stub = ExampleServiceStub(channel)
+        response = await stub.do_thing(names=['a', 'b', 'c'])
+        assert betterproto.serialized_on_wire(response)
+        assert response.names == ['a', 'b', 'c']