tools: do not lint commit message if var undefined

Check that $TRAVIS_COMMIT_RANGE is set before trying to lint commit
messages in Travis CI.

Refs: #23572 (comment)
Refs: #22842 (comment)

.travis.yml

@@ -13,7 +13,7 @@ matrix:
       script:
         - make lint
         # Lint the first commit in the PR.
-        - git log $TRAVIS_COMMIT_RANGE --pretty=format:'%h' --no-merges | tail -1 | xargs npx core-validate-commit --no-validate-metadata
+        - \[ -z "$TRAVIS_COMMIT_RANGE" \] || git log $TRAVIS_COMMIT_RANGE --pretty=format:'%h' --no-merges | tail -1 | xargs npx core-validate-commit --no-validate-metadata
     - name: "Test Suite"
       install:
         - ./configure