Fix watch to return OK when no block given

This was changed in redis-rb 3.0.4.

Change-Id: I8ccf72c309aac753186924d91d670387c3cb0dc5
Reviewed-on: https://gerrit.causes.com/23972
Tested-by: Lann Martin <lann@causes.com>
Reviewed-by: Adam Olsen <adam.olsen@causes.com>