Remove useless check.

Author: WaVEV

django_mongodb/compiler.py

@@ -461,7 +461,7 @@ def get_combinator_queries(self):
         # Raise EmptyResultSet if all the combinator queries are empty.
         if not parts:
             raise EmptyResultSet
-        combinator_pipeline = parts.pop(0)[0].get_pipeline() if parts else None
+        combinator_pipeline = parts.pop(0)[0].get_pipeline()
         if self.query.combinator == "union":
             for part, collection in parts:
                 combinator_pipeline.append(