sdiff in ardb is incompatible with Redis

[srikumarks]

The redis sdiff has the signature sdiff key1 [key2...] meaning only one key needs to be provided and keys that aren't present are considered empty sets. Ardb requires at least two keys to be provided or else it errors out with "incorrect number of arguments". Ardb does correctly treat absent keys as empty sets.

Ref: https://redis.io/commands/sdiff
ardb release: 0.9.7