[8.14] [CI] fix unbound variable issue (#188454) (#188456)

# Backport

This will backport the following commits from `main` to `8.14`:
- [[CI] fix unbound variable issue
(#188454)](#188454)

<!--- Backport version: 9.4.3 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

<!--BACKPORT [{"author":{"name":"Alex
Szabo","email":"[email protected]"},"sourceCommit":{"committedDate":"2024-07-16T16:11:11Z","message":"[CI]
fix unbound variable issue (#188454)\n\n## Summary\r\nLast fix on
#188027","sha":"9d43b39f47433f4f635105872886f77e912398e7","branchLabelMapping":{"^v8.16.0$":"main","^v(\\d+).(\\d+).\\d+$":"$1.$2"}},"sourcePullRequest":{"labels":["release_note:skip","skip-ci","backport:auto-version","v8.15.0","v8.16.0","v8.14.4"],"title":"[CI]
fix unbound variable
issue","number":188454,"url":"https://github.com/elastic/kibana/pull/188454","mergeCommit":{"message":"[CI]
fix unbound variable issue (#188454)\n\n## Summary\r\nLast fix on
#188027","sha":"9d43b39f47433f4f635105872886f77e912398e7"}},"sourceBranch":"main","suggestedTargetBranches":["8.15","8.14"],"targetPullRequestStates":[{"branch":"8.15","label":"v8.15.0","branchLabelMappingKey":"^v(\\d+).(\\d+).\\d+$","isSourceBranch":false,"state":"NOT_CREATED"},{"branch":"main","label":"v8.16.0","branchLabelMappingKey":"^v8.16.0$","isSourceBranch":true,"state":"MERGED","url":"https://github.com/elastic/kibana/pull/188454","number":188454,"mergeCommit":{"message":"[CI]
fix unbound variable issue (#188454)\n\n## Summary\r\nLast fix on
#188027","sha":"9d43b39f47433f4f635105872886f77e912398e7"}},{"branch":"8.14","label":"v8.14.4","branchLabelMappingKey":"^v(\\d+).(\\d+).\\d+$","isSourceBranch":false,"state":"NOT_CREATED"}]}]
BACKPORT-->

Co-authored-by: Alex Szabo <[email protected]>

.buildkite/scripts/steps/artifacts/publish.sh

@@ -62,7 +62,7 @@ if [[ "$BUILDKITE_BRANCH" == "$KIBANA_BASE_BRANCH" ]] || [[ "${DRY_RUN:-}" =~ ^(
   download_artifact beats_manifest.json /tmp --build "${KIBANA_BUILD_ID:-$BUILDKITE_BUILD_ID}"
   export BEATS_MANIFEST_URL=$(jq -r .manifest_url /tmp/beats_manifest.json)
 
-  if [[ "$DRY_RUN" =~ ^(1|true)$ ]]; then
+  if [[ "${DRY_RUN:-}" =~ ^(1|true)$ ]]; then
       docker run --rm \
         --name release-manager \
         -e VAULT_ADDR \