fix(fastrun): fallback when redis config is empty

GitOrigin-RevId: b61630d00723b73477f57ad1b4365d22ba9bf61a

imperative/python/megengine/utils/persistent_cache.py

@@ -34,6 +34,7 @@ class PersistentCacheOnServer(_PersistentCache):
                     "in-file cache".format(exc)
                 )
             else:
+                if redis_config is not None:
                     self.add_config(
                         "redis",
                         redis_config,