[FIX] core: write terms to en_US if not activated

Before this commit:
when en_US is not activated and the user changes terms slightly for
ir.ui.view.arch, the new term will be treated as a typo fix or style change, and
won't be populated to other languages. As a result, in the form view, arch_base
field which displays the en_US translation of the arch_db will still be the
content before the change(wrong and strange).

After this commit:
when write a model_terms field when en_US is not activated, its en_US value will
always be overwritten.

opw-3265418

closes odoo/odoo#121253

Signed-off-by: Raphael Collet <rco@odoo.com>