Save user-entered amount in invoice. fixes #8252.

Note that this allows users to save invoices that have an empty
amount, which is not allowed by the Qt GUI. Qt will complain at
pay time about empty amount if a lightning invoice without amount
is saved. With onchain invoices, Qt will create an onchain tx with
a zero output.

electrum/gui/qml/qeinvoice.py

@@ -313,8 +313,6 @@ class QEInvoiceParser(QEInvoice):
 
         self.set_lnprops()
 
-        self.canSave = True
-
         self.determine_can_pay()
 
         self.invoiceChanged.emit()
@@ -339,6 +337,7 @@ class QEInvoiceParser(QEInvoice):
 
     def determine_can_pay(self):
         self.canPay = False
+        self.canSave = False
         self.userinfo = ''
 
         if not self.amountOverride.isEmpty:
@@ -346,6 +345,8 @@ class QEInvoiceParser(QEInvoice):
         else:
             amount = self.amount
 
+        self.canSave = True
+
         if amount.isEmpty: # unspecified amount
             return
 
@@ -652,6 +653,8 @@ class QEUserEnteredPayment(QEInvoice):
             self.validationError.emit('recipient', _('Invalid Bitcoin address'))
             return
 
+        self.canSave = True
+
         if self._amount.isEmpty:
             self.validationError.emit('amount', _('Invalid amount'))
             return
@@ -659,7 +662,6 @@ class QEUserEnteredPayment(QEInvoice):
         if self._amount.isMax:
             self.canPay = True
         else:
-            self.canSave = True
             if self.get_max_spendable_onchain() >= self._amount.satsInt:
                 self.canPay = True