model: Make message field optional in Release model

A release may have an empty message, for instance those derived from
a Mercurial repository.

So make that field optional to avoid type validation errors.

swh/model/model.py

@@ -371,7 +371,7 @@ class Release(BaseModel, HashableObject):
         type=bytes,
         validator=type_validator())
     message = attr.ib(
-        type=bytes,
+        type=Optional[bytes],
         validator=type_validator())
     target = attr.ib(
         type=Optional[Sha1Git],