performance/md-cache: Guard against null dict

BUG: 1114677
Change-Id: Ica4f4ad97d7d1edc3e48e7f1a6ec70b14acffc66
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/8205
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>