Fix handling of FakePipeline.execute when no commands queued

Closes #217.

fakeredis.py

@@ -2324,6 +2324,8 @@ def __len__(self):
 
     def execute(self, raise_on_error=True):
         """Run all the commands in the pipeline and return the results."""
+        if not self.commands:
+            return []
         try:
             if self.watching:
                 mismatches = [

test_fakeredis.py

@@ -2848,6 +2848,15 @@ def test_pipeline_length(self):
         p.set('baz', 'quux').get('baz')
         self.assertEqual(2, len(p))
 
+    def test_pipeline_no_commands(self):
+        # redis-py's execute is a nop if there are no commands queued,
+        # so it succeeds even if watched keys have been changed.
+        self.redis.set('foo', '1')
+        p = self.redis.pipeline()
+        p.watch('foo')
+        self.redis.set('foo', '2')
+        self.assertEqual(p.execute(), [])
+
     def test_key_patterns(self):
         self.redis.mset({'one': 1, 'two': 2, 'three': 3, 'four': 4})
         self.assertItemsEqual(self.redis.keys('*o*'),