Empty original title in class-walker-nav-menu-edit.php custom menu item type

[grapplerulrich]

When using a custom menu-item-type like line_break, "Original:" without a title is shown after adding the menu item to the menu.

The reason for this is that $original_title is originally defined as a empty string but the check later on is $original_title !== false.

My fix for this would be to define $original_title = false in the beginning

[swissspidy]

Just for completness and better understanding, 36729.diff​ adds a test for that case.

[ocean90]

Introduced in [14287] and [18426].

[ocean90]

In 37953:

Nav Menus: In Walker_Nav_Menu_Edit::start_el() initialize $original_title with false.

Prevents displaying "Original:" without a title when adding a menu item without a title.

Props grapplerulrich, swissspidy.
Fixes #36729.