Use empty alt tag for avatars since there is always the name next to it

Signed-off-by: Joas Schilling <coding@schilljs.com>

core/js/jquery.avatar.js

@@ -141,5 +141,6 @@
 		img.width = size;
 		img.height = size;
 		img.src = url;
+		img.alt = '';
 	};
 }(jQuery));