fix(editor): clearing the foreign amount clears it in Firefly

Firefly keeps a foreign amount sent as null on update and only drops it
for "0".

src/lib/components/editor/model.ts

@@ -364,15 +364,20 @@ function splitPayload(form: Form, split: SplitForm, update: boolean) {
     return body;
   }
   const foreign = hasForeign(split) && split.foreignAmount.trim() !== "";
+  let foreignAmount = update ? "0" : null;
+  if (foreign) {
+    foreignAmount = amountString(
+      split.foreignAmount,
+      split.foreignCurrency?.decimals ?? 2
+    );
+  }
   return {
     ...body,
     ...accountFields("source", sideOf(form, split, "source")),
     ...accountFields("destination", sideOf(form, split, "destination")),
     amount: amountString(split.amount, split.currency?.decimals ?? 2),
     currency_code: split.currency?.code,
-    foreign_amount: foreign
-      ? amountString(split.foreignAmount, split.foreignCurrency?.decimals ?? 2)
-      : null,
+    foreign_amount: foreignAmount,
     foreign_currency_code: foreign ? split.foreignCurrency?.code : null,
   };
 }