test/frontend tag selection fix for nightly/weekly

PRODX-36871

Change-Id: I9989b90bb0815fe488a3e3e8aed106b2e1997418
diff --git a/src/com/mirantis/mk/KaasUtils.groovy b/src/com/mirantis/mk/KaasUtils.groovy
index 84dcc14..f693c27 100644
--- a/src/com/mirantis/mk/KaasUtils.groovy
+++ b/src/com/mirantis/mk/KaasUtils.groovy
@@ -1500,8 +1500,8 @@
  */
 def getImageTag(version, isChanged, imageName) {
     def common = new com.mirantis.mk.Common()
-    def latestTag = '' 
-    if (!(env.GERRIT_EVENT_TYPE in ['change-merged', 'ref-updated']) && isChanged) {
+    def latestTag = ''
+    if (env.GERRIT_EVENT_TYPE && !(env.GERRIT_EVENT_TYPE in ['change-merged', 'ref-updated']) && isChanged) {
         latestTag = version
     } else {
         if (env.GERRIT_EVENT_TYPE == 'ref-updated') {