remove "main-menu" div when the project menu is empty
On some pages, like Home or My page or plugin using main menu entries the project menu is empty but the div html is still present.
It's a problem when you a background image in CSS, because you see the background image without the menu items. Also its a problem if you use contect menu slider as done in plugin redmine sidebar hide where additional space is made availaibe if collapsed
Could you remove this div or add a CSS class to identify an empty menu so i am able to unset css main.margin-left property, for view here no project context and aka no project menu entries are available.
The fix #3259 doesn't fix the problem, because i still have empty "main-menu" divs in my rendered site
<div id="main-menu"> </div>