Use non-empty issue_message

Author: swissspidy

.github/workflows/reusable-welcome-new-contributors.yml

@@ -13,7 +13,9 @@ jobs:
         uses: actions/first-interaction@v3
         with:
           repo_token: ${{ secrets.GITHUB_TOKEN }}
-          issue_message: ""
+          # Not currently used but must be supplied.
+          issue_message: |
+            Hello! 👋
           pr_message: |
             Hello! 👋