Unable to set default_currency to nil

[sirwolfgang]

Noticed an issue where in our system an edge case would cause the default currency of USD to get applied. Generally we want to be strict, and not risk a default value. When I attempted to set the default_currency to nil this started to error.

Based on this message I found while debugging [WARNING] The default currency will change from 'USD' to 'nil' in the next major release., it doesn't sound like it's intended to prevent the removal of a default.

I've prepared two PRs that make the required adjustments to allow nil to be used as a default value, that triggers NoCurrency error.

This will allow strictly defined systems to operate without risk of unexpected currencies.