Redis fetch without names returns {}

When trying to call mget in Redis without any
parameters, a Redis error is thrown. To avoid
this, we circumvent Redis entirely when there
are no key names given.

activesupport/lib/active_support/cache/redis_cache_store.rb

@@ -361,6 +361,7 @@ def read_multi_entries(names, _options)
         def read_multi_mget(*names)
           options = names.extract_options!
           options = merged_options(options)
+          return {} if names == []
 
           keys = names.map { |name| normalize_key(name, options) }
 

activesupport/test/cache/stores/redis_cache_store_test.rb

@@ -140,6 +140,12 @@ def test_fetch_multi_uses_redis_mget
       end
     end
 
+    def test_fetch_multi_without_names
+      assert_not_called(@cache.redis, :mget) do
+        @cache.fetch_multi() { }
+      end
+    end
+
     def test_increment_expires_in
       assert_called_with @cache.redis, :incrby, [ "#{@namespace}:foo", 1 ] do
         assert_called_with @cache.redis, :expire, [ "#{@namespace}:foo", 60 ] do