Redis [NEW] Make RDB and AOF callback functions optional

The problem/use-case that the feature addresses

Some custom data types implemented as Redis modules may not need or be able to persist data to the Redis database.

Description of the feature

I propose to make the RDB and AOF callback functions optional, so Redis ignores these modules when trying to load or save data. Currently, these callback functions are required and if missing (e.g. are set to NULL in the data type structure), Redis simply crashes (it seems like there is no validation).

Alternatives you've considered

I currently have a module that does not persist data and my RDB and AOF callback functions do nothing. The problem with this is that an entry for my module is still included in the RDB and if I do not load the module, I cannot load other data from the RDB (e.g. for other data types). Another possibility would be to skip database entries from a module that is not loaded (but still keep the data), but I think this would be a feature in itself and useful in cases beyond the scenario described here.

Additional information

We could make the RDB and AOF callback functions optional in a way that would make it valid to leave them all as NULL or pointing to functions, but other combinations would be considered invalid and would result in an error when loading the module.