Convert gr-repo-header_test.js to typescript

Change-Id: I4ea6323198c1bbf0c44ea8629c4ca7ab9ca4f13b
diff --git a/polygerrit-ui/app/api/rest-api.ts b/polygerrit-ui/app/api/rest-api.ts
index 578dc58..be8aaff 100644
--- a/polygerrit-ui/app/api/rest-api.ts
+++ b/polygerrit-ui/app/api/rest-api.ts
@@ -1018,7 +1018,7 @@
   /** The link URL. */
   url: string;
   /** URL to the icon of the link. */
-  image_url: string;
+  image_url?: string;
 }
 
 /**