wp_nav_menu: Don't return markup, when no menu item is set

[obenland]

Having assigned an empty menu to a location can be assumed to be the user's way of "disabling" the menu.

When there are no items, there shouldn't be a menu, hence there shouldn't be any extra menu markup.
Rendering markup nonetheless could lead to issues when menu wrappers are styled (as they are in Twenty Twelve) and left empty.

Currently out put is:

<div class="menu-menu-container"><ul id="menu-menu" class="menu"></ul></div>

Related: #21562

[nacin]

I can get behind this.

[lancewillett]

+1

[nacin]

In [21868]:

If a nav menu has no menu items, print nothing (instead of empty container markup). props obenland. fixes #21576.

[DrewAPicture]

Related: #24035

[nacin]

In 24634:

If a nav menu has no items, wait until after the wp_nav_menu_items filter before deciding whether to print nothing.

see [21868] for original commit. see #21576.
see #24035 for trunk.