Dont set group for optional fields (#528)

Fixes #523

src/betterproto/__init__.py

@@ -909,7 +909,7 @@ class Message(ABC):
             # Note that proto3 field presence/optional fields are put in a
             # synthetic single-item oneof by protoc, which helps us ensure we
             # send the value even if the value is the default zero value.
-            selected_in_group = bool(meta.group)
+            selected_in_group = bool(meta.group) or meta.optional
 
             # Empty messages can still be sent on the wire if they were
             # set (or received empty).

src/betterproto/plugin/models.py

@@ -385,7 +385,10 @@ def is_oneof(proto_field_obj: FieldDescriptorProto) -> bool:
         us to tell whether it was set, via the which_one_of interface.
     """
 
-    return which_one_of(proto_field_obj, "oneof_index")[0] == "oneof_index"
+    return (
+        not proto_field_obj.proto3_optional
+        and which_one_of(proto_field_obj, "oneof_index")[0] == "oneof_index"
+    )
 
 
 @dataclass