Changes

fix(pipelines): handle missing pulls in merged tidb jobs (#4579)

## What changed

Fix two merged TiDB pipelines that were broken by #4544 in
postsubmit/replay scenarios where `JOB_SPEC.refs.pulls` is absent.

- `pipelines/pingcap/tidb/latest/merged_integration_br_test.groovy`
-
`pipelines/pingcap/tidb/latest/merged_integration_lightning_test.groovy`

Both files already compute a safe `PR_TITLE`, but `OCI_TAG_PD` was
changed to read `REFS.pulls[0].title` directly. This restores the safe
`PR_TITLE` usage so merged jobs no longer throw NPE before scheduling.

## Verification

- Jenkinsfile validator
  - `merged_integration_br_test.groovy`: passed
  - `merged_integration_lightning_test.groovy`: passed
- Replay
- Replayed `merged_integration_lightning_test #168` with the local
script
  - New replay build: `merged_integration_lightning_test #170`
- Console no longer fails at `WorkflowScript:11`; it proceeds through
`computeBranchFromPR(...)` and creates the Jenkins pod
- Replay URL:
https://prow.tidb.net/jenkins/job/pingcap/job/tidb/job/merged_integration_lightning_test/170/
 (commit: d5669de)