Added a missing nullcheck in stats converter

There was a missing null check in
FlowStatsResponseConvertor

This pre-existing bug was uncovered when a bug was fixed in the
setCookie() method that caused it to stop inappropriately accepting
nulls.

Change-Id: I1a1bcc8f9de1cc49573b468726418520081159fb
Signed-off-by: Ed Warnicke <eaw@cisco.com>