[FIX] prevent entering a blank amount_currency in an account.move.line

This is a fix for https://github.com/odoo/odoo/issues/4242 
Upstream PR: https://github.com/odoo/odoo/pull/4457
That's how it is done in the decimal precision module: https://github.com/OCA/OCB/blob/8.0/addons/decimal_precision/decimal_precision.py#L74

I do not know why it is like that in there either :)