fix: More Offline Currency Payout Issues

- 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

src/backend/currency/currency-manager.ts

@@ -263,7 +263,7 @@ class CurrencyManager {
         for (const currency of currencies) {
             let basePayout = currency.payout;
             if (!connectionManager.streamerIsOnline()) {
-                if (currency.offline == null || currency.offline === 0) {
+                if (currency.offline == null || (currency.offline as unknown) === "" || currency.offline === 0) {
                     continue;
                 }