When adding a Custom Link and leaving the "Link Text" field empty, WordPress used to set a default fallback text: "Menu Item".
The changes in [36379] broke this behavior making the fallback text: `(Pending)`, with a leading space.
Pending major refactoring of the Menus page (which is going to use a block-based user interface) this change just restores the original behavior by adding the fallback text to the related AJAX response.
Props christophherr, Fencer04, thakkarhardik, backermann1978, audrasjb.
Merges [45727] to the 5.2 branch.
Fixes#38415.
Built from https://develop.svn.wordpress.org/branches/5.2@45828
git-svn-id: http://core.svn.wordpress.org/branches/5.2@45639 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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