Fix None instance

Author: tonibofarull

aredis_om/model/model.py

@@ -1205,6 +1205,8 @@ def to_string(s):
         offset = 1  # The first item is the count of total matches.
 
         for i in range(1, len(res), step):
+            if res[i + offset] is None:
+                continue
             fields = dict(
                 zip(
                     map(to_string, res[i + offset][::2]),