Fix typechecker compatiblity checks in server streaming methods (#413)

src/betterproto/templates/template.py.j2

@@ -179,6 +179,9 @@ class {{ service.py_name }}Base(ServiceBase):
 
         {% endif %}
         raise grpclib.GRPCError(grpclib.const.Status.UNIMPLEMENTED)
+        {% if method.server_streaming %}
+        yield {{ method.py_output_message_type }}()
+        {% endif %}
 
     {% endfor %}