nixos.redis: Fix disabling of RDB persistence.

I was under the impression that setting `services.redis.servers.<name>.save = []` would disable RDB persistence as no schedule would mean no persistence. However since the code did not handle this case specially it actually results in no `save` setting being written and the internal Redis default is used.

This patch handles the empty case to disable RDB persistence.

Disabling RDB persistence is useful in a number of scenarios:

1. Using Redis in a pure-cache mode where persistence is not desired.
2. When using the (generally superior) AOF persistence mode this file is never read so there is little point to writing it.
3. When saving is handled manually

For more information see https://redis.io/docs/manual/persistence/

This is a breaking change as the user may have been relying on `[]` using Redis defaults. However I believe that updating the behaviour for the next release is beneficial as IMHO it is less surprising and does what the user would expect. I have added release notes to warn about this change.

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