rpoplpush: don't lpush if the rpop was nil

fixes #54

Change-Id: Ief4390e909e45f04d17384be52aaebe536843479
Reviewed-on: https://gerrit.causes.com/20665
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Aiden Scandella <aiden@causes.com>
add spec for empty source list for rpoplpush

Change-Id: Ib2832025c554d0b777ec8af5f1fddaa42e839933
Reviewed-on: https://gerrit.causes.com/20666
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Aiden Scandella <aiden@causes.com>