Networks and Sites: Don't display an empty Menu Settings section in network...

Networks and Sites: Don't display an empty Menu Settings section in network admin if there are no items.

This accounts for returning an empty array from the `mu_menu_items` filter.

Props dlh.
Fixes #49977.
Built from https://develop.svn.wordpress.org/trunk@47616


git-svn-id: https://core.svn.wordpress.org/trunk@47391 1a063a9b-81f0-0310-95a4-ce76da25c4cd