fix #336: detecting relationship as nullable=False

src/marshmallow_sqlalchemy/convert.py

@@ -348,7 +348,7 @@ def _add_relationship_kwargs(self, kwargs, prop):
         nullable = True
         for pair in prop.local_remote_pairs:
             if not pair[0].nullable:
-                if prop.uselist is True:
+                if not self.DIRECTION_MAPPING[prop.direction.name]:
                     nullable = False
                 break
         kwargs.update({"allow_none": nullable, "required": not nullable})