build(ci): only validate branch name on pull request (#6190)

## Problem
It appears that on `push` events the `head_ref` is an empty string,
which is not a valid branch name.

## Solution
- Only validate branch names on pull_request events, and use event info
since its more consistent.

---

- Treat all work as PUBLIC. Private `feature/x` branches will not be
squash-merged at release time.
- Your code changes must meet the guidelines in
[CONTRIBUTING.md](https://github.com/aws/aws-toolkit-vscode/blob/master/CONTRIBUTING.md#guidelines).

License: I confirm that my contribution is made under the terms of the
Apache 2.0 license.

Author: Hweinstock

.github/workflows/node.js.yml

@@ -32,9 +32,10 @@ jobs:
             - uses: actions/setup-node@v4
               with:
                   node-version: '20'
-            - name: Check Branch title
+            - name: Validate Branch name
+              if: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.ref != ''}}
               env:
-                  BRANCH_NAME: ${{ github.head_ref }}
+                  BRANCH_NAME: ${{ github.event.pull_request.head.ref }}
               run: |
                   node "$GITHUB_WORKSPACE/.github/workflows/lintbranch.js" run "$BRANCH_NAME"
             - name: Check PR title