Fixed bug in IronCacheBackend that could cause crashing of celery worker...

When celery pickles/unpickles the results backend the url can sometimes be None which resulted in a crash even though the pickling would restore all of the appropriate variables.

This change simple skips over parsing the url where a crash was occurring if the url passed in was None. This theoretically could be dangerous as you could have a corrupt backend but many of celery's built in backends use this same method.