text input: fix disabling of action logic when there is a menu assigned

The "menu.source" check added by 38b908a is always true even
for an empty url => use "menu.item" instead.

Change-Id: Ic9004dff18c848a720ea1c414b2921158a367778
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@theqtcompany.com>