Multisite: 'mu_menu_items' filter leaves empty table on Network Settings

[dlh]

As a developer maintaining a multisite network, I'd like to prevent super administrators from enabling any administration menus on individual sites, such as "Plugins." I can do this with:

add_filter( 'mu_menu_items', '__return_empty_array' );

However, the Menu Settings heading and empty settings table still appear on the Network Settings screen. See the attached screenshot.

The attached patch would check for available menus after applying mu_menu_items and before rendering the markup.

[SergeyBiryukov]

In 47616:

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.