Fix inconsistency between `rpoplpush` and `rpop`
Fix inconsistency between `rpoplpush` and `rpop`

`rpoplpush` (also `brpoplpush`) did not have the default empty list behaviour, causing "IndexError: pop from empty list" errors.

Pretty sure they should behave the same as `rpop` and `brpop` in this regard.
sam-untapt/fakeredis,OnBeep/fakeredis,ze-phyr-us/fakeredis,fatelei/fakeredis,Tinche/fakeredis