Fix set_many to work with empty dict

Instead of raising an exception it should just return without doing anything.

redis_cache/backends/base.py

@@ -318,6 +318,9 @@ class BaseRedisCache(BaseCache):
         raise NotImplementedError
 
     def _set_many(self, client, data):
+        # Only call mset if there actually is some data to save
+        if not data:
+            return True
         return client.mset(data)
 
     def set_many(self, data, timeout=DEFAULT_TIMEOUT, version=None):

tests/testapp/tests/base_tests.py

@@ -345,6 +345,11 @@ class BaseRedisTestCase(SetupMixin):
         self.assertEqual(self.cache.get("key1"), "spam")
         self.assertEqual(self.cache.get("key2"), "eggs")
 
+    def test_set_many_works_with_empty_dict(self):
+        # This test passes if no exception is raised
+        self.cache.set_many({})
+        self.cache.set_many({}, version=2)
+
     def test_set_many_expiration(self):
         # set_many takes a second ``timeout`` parameter
         self.cache.set_many({"key1": "spam", "key2": "eggs"}, 1)