[FIX] web_editor, web: fix traceback with drag & drop when no header

Commit [1] introduced the bug by relying on the fact an element with
id=top is in the edited DOM. As the website header can be disabled, this
induced a crash as soon as the header was indeed disabled. Also, the
editor is not meant to edit the website only so we cannot rely at all
on the presence of a #top element.

[1]: https://github.com/odoo/odoo/commit/6df67a28c2f113572acd4a3db19ab6b1bd37e7da



closes odoo/odoo#54409

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>