Skip empty description in pr_labels.yaml workflow (#1117)

.github/workflows/pr_labels.yaml

@@ -31,7 +31,8 @@ jobs:
               ['* Backward incompatible change', 'backward-incompatible'],
               ['* Documentation', 'documentation'],
               ['* Not for changelog', 'not-for-changelog']
-            ];          
+            ];
+            if (!description) return;
             for (let pair of mapping) {
               if (!description.includes(pair[0])) continue;
               try {