fix: [ci-fix] skip staging job when secrets are not configured

The staging job fails because SUPABASE_STAGING_PROJECT_ID and
SUPABASE_STAGING_DB_PASSWORD secrets don't exist yet. Add a guard
to skip the job when the secret is empty, so CI passes until the
staging Supabase project is created.

Co-authored-by: Ona <no-reply@ona.com>

Author: zacharias-ona

.github/workflows/deploy-migrations.yml

@@ -15,7 +15,7 @@ jobs:
   # PR check: reset staging DB then push migrations for real execution
   # ---------------------------------------------------------------------------
   staging:
-    if: github.event_name == 'pull_request'
+    if: github.event_name == 'pull_request' && secrets.SUPABASE_STAGING_PROJECT_ID != ''
     runs-on: ubuntu-latest
     # Only one staging validation at a time — later PRs queue behind earlier ones
     concurrency: