PolyGerrit: Fix undefined url in gr-dropdown

If link.url is undefined lets return it as '' so no url is applied.

Bug: Issue 6352
Change-Id: I9492e48b14e4387cbdbe87c562ffa4966323948c

polygerrit-ui/app/elements/shared/gr-dropdown/gr-dropdown.js

@@ -88,6 +88,9 @@
     },
 
     _computeLinkURL: function(link) {
+      if (typeof link.url === 'undefined') {
+        return '';
+      }
       if (link.target) {
         return link.url;
       }