handle unbound SKIP_CLUSTER_UPGRADE_CHECK variable

Author: laverya

e2e/scripts/playwright.sh

@@ -17,7 +17,9 @@ main() {
     export DR_AWS_SECRET_ACCESS_KEY="$7"
   elif [ "$test_name" == "deploy-upgrade" ]; then
     export APP_UPGRADE_VERSION="$2"
-    export SKIP_CLUSTER_UPGRADE_CHECK="$3"
+    if [ -n "$3" ]; then
+      export SKIP_CLUSTER_UPGRADE_CHECK="$3"
+    fi
   fi
 
   export BASE_URL="http://10.0.0.2:30001"