Promotion: fix promotion check

the script to check if the change is allowed to promote was not
considering the set -u, so when ZUUL_CHANGES was empty CHANGES[@] was
unbound.
Adding fallback to empty for the variable in the loop

Change-Id: I16fc10358147b2bd54502d15f7e96cc94cccac63

toci_instack_ovb.sh

@@ -120,7 +120,7 @@ CHANGES=( ${ZUUL_CHANGES//^/ } )
 NUM_CHANGES=${#CHANGES[@]}
 
 OTHER_PROJECTS="no"
-for change in ${CHANGES[@]}; do
+for change in ${CHANGES[@]-}; do
     if [[ ! "$change" =~ 'openstack-infra/tripleo-ci' ]]; then
         OTHER_PROJECTS="yes"
     fi