[FIX] account: crash when unlink taxes
[FIX] account: crash when unlink taxes

If `get_default` returns an empty value (`None`), the `set` instruction
crashes.

Backport of 26d1a3884b3ccb2918acccb69071f3426824f862

Closes #15116
opw-702597

Elico-Corp/odoo_OCB,ygol/odoo,laslabs/odoo,ygol/odoo,bplancher/odoo,hip-odoo/odoo,bplancher/odoo,ygol/odoo,laslabs/odoo,bplancher/odoo,dfang/odoo,hip-odoo/odoo,hip-odoo/odoo,ygol/odoo,laslabs/odoo,ygol/odoo,dfang/odoo,hip-odoo/odoo,hip-odoo/odoo,bplancher/odoo,dfang/odoo,bplancher/odoo,dfang/odoo,bplancher/odoo,laslabs/odoo,Elico-Corp/odoo_OCB,Elico-Corp/odoo_OCB,laslabs/odoo,dfang/odoo,hip-odoo/odoo,Elico-Corp/odoo_OCB,ygol/odoo,laslabs/odoo,Elico-Corp/odoo_OCB,ygol/odoo,dfang/odoo,Elico-Corp/odoo_OCB