guard against coercing `undefined` to a string

visible in the author's tooltip when author marks were enabled
for the document, but hidden by the local user

www/code/markers.js

@@ -46,7 +46,7 @@ define([
             inclusiveRight: uid === Env.myAuthorId,
             css: "background-color: " + rgba,
             attributes: {
-                title: Env.opacity ? Messages._getKey('cba_writtenBy', [name]) : undefined,
+                title: Env.opacity ? Messages._getKey('cba_writtenBy', [name]) : '',
                 'data-type': 'authormark',
                 'data-uid': uid
             }