loginDialog: Set null user when asking for username

When going through the username + password login flow, set
the username to 'null' to update the user avatar.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/922

js/gdm/loginDialog.js

@@ -913,6 +913,7 @@ var LoginDialog = GObject.registerClass({
     }
 
     _askForUsernameAndBeginVerification() {
+        this._authPrompt.setUser(null);
         this._authPrompt.setQuestion(_("Username: "));
 
         this._showRealmLoginHint(this._realmManager.loginFormat);