Fix is_set for optional proto3 fields

src/betterproto/__init__.py

@@ -1319,7 +1319,12 @@ class Message(ABC):
         :class:`bool`
             `True` if field has been set, otherwise `False`.
         """
-        return self.__raw_get(name) is not PLACEHOLDER
+        default = (
+            PLACEHOLDER
+            if not self._betterproto.meta_by_field_name[name].optional
+            else None
+        )
+        return self.__raw_get(name) is not default
 
 
 def serialized_on_wire(message: Message) -> bool: