fix: Offline Currency Data Typing

- 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.

src/gui/app/directives/modals/currency/addOrEditCurrency/addOrEditCurrency.js

@@ -56,6 +56,10 @@
                     return;
                 }
 
+                if (!$ctrl.currency.offline || $ctrl.currency.offline === "") {
+                    $ctrl.currency.offline = undefined;
+                }
+
                 logger.debug($ctrl.currency);
 
                 const action = $ctrl.isNewCurrency ? "add" : "update";