Money.ToString() fails for all currency with NotApplicable (-1) decimal places

[ptjhuang]

When currency is inferred from the invariant culture, which has XDR as the currency code, ToString() throws because it's trying to setup a NumberFormatInfo with -1 DecimalDigits.

ToString() shouldn't throw, as per: https://docs.microsoft.com/en-us/dotnet/api/system.object.tostring?view=netcore-3.1#notes-to-inheritors

Either don't allow Money to be created with XDR, or use a workable value for DecimalDigits in CurrencyRegistry.

This will likely cause a problem for developers who don't need multicurrency support, but have a CI process using dotnet core SDK docker images where the default culture is the invariant culture.

As a side note, Unicode chose to use 2dp, even though ISO publication is N/A. Practically XDR probably needs decimal places. https://www.unicode.org/cldr/charts/latest/supplemental/detailed_territory_currency_information.html