Add conditional comment creation for diff

.github/workflows/pyrefly-diff-comment.yml

@@ -79,10 +79,11 @@ jobs:
             const body = diff.trim()
               ? '### Pyrefly Diff\n<details>\n<summary>base → PR</summary>\n\n```diff\n' + diff + '\n```\n</details>'
               : '### Pyrefly Diff\nNo changes detected.';
-
-            await github.rest.issues.createComment({
-              issue_number: prNumber,
-              owner: context.repo.owner,
-              repo: context.repo.repo,
-              body,
-            });
+            if (diff.trim()) {
+              await github.rest.issues.createComment({
+                issue_number: prNumber,
+                owner: context.repo.owner,
+                repo: context.repo.repo,
+                body,
+              });
+            }