Avoid spurious cache miss when query is empty (vijaykatam#30)

The lines after `if not result_set:` are executed (as if there had been a cache miss) if the key was found but the resulting set was empty (`False` in a boolean context). This changes the line for a explicit comparison with `None` (the response if there's a cache miss).

Author: Laski

django_cache_manager/cache_manager.py

@@ -47,7 +47,7 @@ def iterator(self):
         except EmptyResultSet:
             return
         result_set = self.cache_backend.get(key)
-        if not result_set:
+        if result_set is None:
             logger.debug('cache miss for key {0}'.format(key))
             result_set = list(super(CachingQuerySet, self).iterator())
             self.cache_backend.set(key, result_set)