Add error handling for fetching runners in runner-fallback action

.github/actions/runner-fallback/action.yml

@@ -40,6 +40,13 @@ runs:
         runners=$(curl -s -H "Authorization: Bearer $TOKEN" \
                         -H "Accept: application/vnd.github+json" \
                         "https://api.github.com/repos/$repo/actions/runners?per_page=100")
+
+        # Check if runners is null or empty
+        if [ -z "$runners" ] || [ "$runners" = "null" ]; then
+          echo "❌ Error: Unable to fetch runners or no runners found." >&2
+          exit 1
+        fi
+
         # Split wanted labels
         IFS=',' read -ra WANT <<<"$PRIMARY"
         online_found=0