Handle unauthenticated users in when computing change list highlight

In I93e61f4c98, the `_computeItemHighlight` method was changed in a way
that didn't account for unauthenticated users. Update the method to
handle this scenario and add a test.

The dashboard view is updated to set null accounts more consistently
with the change list view.

Change-Id: Id33808505ffb6c3f722a963d8e27dd8dba112dec
diff --git a/polygerrit-ui/app/elements/change-list/gr-dashboard-view/gr-dashboard-view.js b/polygerrit-ui/app/elements/change-list/gr-dashboard-view/gr-dashboard-view.js
index 7c04c1f..cce291b 100644
--- a/polygerrit-ui/app/elements/change-list/gr-dashboard-view/gr-dashboard-view.js
+++ b/polygerrit-ui/app/elements/change-list/gr-dashboard-view/gr-dashboard-view.js
@@ -72,7 +72,7 @@
     properties: {
       account: {
         type: Object,
-        value() { return {}; },
+        value: null,
       },
       /** @type {{ selectedChangeIndex: number }} */
       viewState: Object,