build: do not lint on non-PR Travis builds

Do not run any linting at all when `TRAVIS_PULL_REQUEST` is `false`.
This would otherwise break Travis CI for `master` and release branches.

Refs: #24030

PR-URL: #24076
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

Author: addaleax

.travis.yml

@@ -11,7 +11,7 @@ matrix:
       script:
         - make lint
         # Lint the first commit in the PR.
-        - \[ "${TRAVIS_PULL_REQUEST}" != "false" \] && PR_ID=${TRAVIS_PULL_REQUEST}; bash tools/lint-pr-commit-message.sh ${PR_ID}
+        - \[ "${TRAVIS_PULL_REQUEST}" != "false" \] && bash tools/lint-pr-commit-message.sh ${TRAVIS_PULL_REQUEST}
     - name: "Test Suite"
       addons:
         apt: