[FIX] account: show inactive warning only when currency is set

before this commit, in active currency warning is shown in the account move form when the currency_field is empty.

after this commit, the inactive currency warning will be shown only if the currency is set and it is inactive.

closes odoo/odoo#110920

Signed-off-by: Brice Bartoletti (bib) <bib@odoo.com>