Fix bug in gr-avatar which wasn't hiding avatars when "hidden" is set

This happened for sites that did not set an avatar. Seems a recent
regression as it works on gerrit 3.3.

Change-Id: I148716b36c3d57e3eb3f1486b9539f1741d7288b
diff --git a/polygerrit-ui/app/elements/shared/gr-avatar/gr-avatar.ts b/polygerrit-ui/app/elements/shared/gr-avatar/gr-avatar.ts
index 80576a7..1d2ccbb 100644
--- a/polygerrit-ui/app/elements/shared/gr-avatar/gr-avatar.ts
+++ b/polygerrit-ui/app/elements/shared/gr-avatar/gr-avatar.ts
@@ -37,6 +37,9 @@
   static get styles() {
     return [
       css`
+        :host([hidden]) {
+          display: none;
+        }
         :host {
           display: inline-block;
           border-radius: 50%;