Qt: allow to save invoices that have no amount

(such invoices are already saved by the QML GUI.)

electrum/invoices.py

@@ -161,7 +161,8 @@ class BaseInvoice(StoredObject):
         Returns an integer satoshi amount, or '!' or None.
         Callers who need msat precision should call get_amount_msat()
         """
-        amount_msat = self.amount_msat
+        # return strictly positive value or None
+        amount_msat = self.amount_msat or None
         if amount_msat in [None, "!"]:
             return amount_msat
         return int(amount_msat // 1000)