Allow empty services (#222)

Fixes issue #220

Author: qria

src/betterproto/templates/template.py.j2

@@ -73,6 +73,8 @@ class {{ service.py_name }}Stub(betterproto.ServiceStub):
     {% if service.comment %}
 {{ service.comment }}
 
+    {% elif not service.methods %}
+    pass
     {% endif %}
     {% for method in service.methods %}
     async def {{ method.py_name }}(self

tests/inputs/config.py

@@ -18,4 +18,5 @@
     "googletypes_service_returns_empty",
     "googletypes_service_returns_googletype",
     "example_service",
+    "empty_service",
 }

tests/inputs/empty_service/empty_service.proto

@@ -0,0 +1,7 @@
+/* Empty service without comments */
+syntax = "proto3";
+
+package empty_service;
+
+service Test {
+}