Fix navigate back to change page with '[' on first diff.

It got broken by js-ts migration.

Bug: Issue 13543

Change-Id: I1d331448dbe9c1da7d58d27d19ea2e00a06925b1

polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.ts

@@ -726,7 +726,7 @@ export class GrDiffView extends KeyboardShortcutMixin(
 
   _navToFile(path: string, fileList: string[], direction: -1 | 1) {
     const newPath = this._getNavLinkPath(path, fileList, direction);
-    if (!newPath?.path) return;
+    if (!newPath) return;
     if (!this._change) return;
     if (!this._patchRange) return;
 
@@ -739,6 +739,7 @@ export class GrDiffView extends KeyboardShortcutMixin(
       return;
     }
 
+    if (!newPath.path) return;
     GerritNav.navigateToDiff(
       this._change,
       newPath.path,