[FIX] web: accept is_html_empty in layout wizard (fixup)

After eb3dcd60cb88f190d6398d7cadfb7852eecf909c is_html_empty was
accepted in document layout and would no longer cause an error when
using "Configure Document Layout" wizard.

If l10n_de is installed, this caused an error itself in "Configure
Document Layout" because of the content of the external_layout_din5008
that itself use the "company" variable if available (and in the given
case, this is a NewId base.document.layout record).

To fix this issue, this commit backports the 16.0 forward-port of the
referenced commit (5f1ad0c1).

opw-3433583

closes odoo/odoo#133447

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>