build: ci failure notifications accidentally skipped (#19631)

Looks like we currently incorrectly skip when the
`CIRCLE_PR_NUMBER` variable is _not_ defined. It
should be the other way around though, and we want
to only skip when this variable is defined/not-null

Author: devversion

scripts/circleci/notify-slack-job-failure.sh

@@ -3,7 +3,7 @@
 # Script that notifies Slack about the currently failing job. This script
 # will be a noop when running for forked builds (i.e. PRs).
 
-if [[ -z "${CIRCLE_PR_NUMBER}" ]]; then
+if [[ -n "${CIRCLE_PR_NUMBER}" ]]; then
   echo "Skipping notification for pull request."
   exit 0
 fi