When no keys are passed to a Redis command that accepts multiple keys,
that's an error. But it's not a cross-slot violation, as there are no
slots. By raising this error we were masking the true
Redis::CommandError underneath, which is bad for two reasons:

1. It's confusing to developers.
2. It leads to a different failure in production to development and
   test.

To elaborate a bit more on the second point: the purpose of the
CrossSlotError is to only be an _additional_ error in development and
test, but it shouldn't _replace_ underlying errors as that can cause
issues like this one.