skip redis initialization if no servers specified

matches behavior of memcache block above

Change-Id: I0baba8d2b55c362403a9e1d3e9499c022c8b039c
Reviewed-on: https://gerrit.instructure.com/39975
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>