don't lpush None from `rpoplpush` where src list was empty
don't lpush None from `rpoplpush` where src list was empty

and same for `brpoplpush`

according to Redis docs, if src list was empty nothing should get pushed and method returns `nil`/`None`
sam-untapt/fakeredis,Tinche/fakeredis,fatelei/fakeredis,OnBeep/fakeredis,ze-phyr-us/fakeredis