Implement Message.__bool__ (#142)

* Implement Message.__bool__ with similar semantics to a collection, such that any value being set on the message (i.e. having a non-default value) make the Message value truthy .

Co-authored-by: nat <n@natn.me>

src/betterproto/__init__.py

@@ -516,6 +516,10 @@ class Message(ABC):
         .. describe:: bytes(x)
 
             Calls :meth:`__bytes__`.
+
+        .. describe:: bool(x)
+
+            Calls :meth:`__bool__`.
     """
 
     _serialized_on_wire: bool
@@ -606,6 +610,14 @@ class Message(ABC):
 
         super().__setattr__(attr, value)
 
+    def __bool__(self) -> bool:
+        """True if the Message has any fields with non-default values."""
+        return any(
+            self.__raw_get(field_name)
+            not in (PLACEHOLDER, self._get_field_default(field_name))
+            for field_name in self._betterproto.meta_by_field_name
+        )
+
     @property
     def _betterproto(self) -> ProtoClassMetadata:
         """