- Currency payouts would stop paying out while offline once any currency
  had a zero offline payout.
- Changed it to keep checking additional currencies.
- Issue crowbartools#2796

- Checking then unchecking the Offline Payout checkbox in the add or
  edit currency dialog results in the offline property being assigned to
  an empty string.
- This conflicts with the type definition of a Currency in
  currency-access.ts.
- So pay special attention to it potentially being an empty string.
- See issue crowbartools#2801

This reverts commit 197fe92.
Going to edit the currency add/edit controller instead. It's better for
everyone.

- Checking then unchecking the offline payout option in an add or edit
  currency modal will assign an empty string to the data model.
- The data model is typed as a Number.
- This typing is exposed to external scripts.
- So trim the null or empty string on addOrEditCurrency controller save
  to prevent a string from getting through.
- See issue crowbartools#2801.

- Remove the empty string offline currency payout upon read.
  - Immediately re-write the file without the poorly-typed value.
- Remove the empty string offline currency payout upon import, before it
  actually gets added or updated.

- currency-access.ts should also catch nulls, while 0's are OK.
- currency-manager.ts is much more concise with a boolean nullish.
- As-is currency.service.js...
- ... which also had two lint warnings in it that got buffed out.

- addOrEditCurrency.js can also use a boolean nullish.
- Buffed out the two arrow-parens lint warnings in there as well.