Menus: Check the correct variable in `Walker_Nav_Menu_Edit::start_el()` when...

Menus: Check the correct variable in `Walker_Nav_Menu_Edit::start_el()` when menu item is a taxonomy term.

Follow-up to [45891].

Props ashour.
Fixes #50415.
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git-svn-id: https://core.svn.wordpress.org/trunk@47843 1a063a9b-81f0-0310-95a4-ce76da25c4cd