have a default prefix, check prefix to prevent downstream issues

[JunAishima]

Having an undefined, or user-set None for prefix will cause problems with the current code. Suggest adding some validation and a default value.

(line 29 of redis_json_dict.py): prefix_len = len(self._prefix)

[danielballan]

Undefined cannot happen, as prefix is a required argument for instantiation.

I considered that the only reasonable default would be "", and by providing no default, the user is forced to choose a prefix and in that moment consider what would best avoid key collisions in their application.