qml: send screen: bip21: fallback to onchain addr if no LN channels

given a bip21 uri that has both onchain addr and bolt11,
if we have LN enabled but no LN channels, auto-fallback to paying onchain

we will have to clean up and unify this logic between GUIs. becoming spaghetti :/
rumour has it, Thomas has a branch? :P

electrum/gui/qml/components/WalletMainView.qml

@@ -232,6 +232,9 @@ Item {
         onValidationWarning: {
             if (code == 'no_channels') {
                 var dialog = app.messageDialog.createObject(app, { text: message })
+                dialog.closed.connect(function() {
+                    restartSendDialog()
+                })
                 dialog.open()
                 // TODO: ask user to open a channel, if funds allow
                 // and maybe store invoice if expiry allows

electrum/gui/qml/qeinvoice.py

@@ -554,10 +554,14 @@ class QEInvoiceParser(QEInvoice):
                     self._logger.debug('flow with LN but not LN enabled AND having bip21 uri')
                     self._validateRecipient_bip21_onchain(bip21)
             else:
-                self.setValidLightningInvoice(lninvoice)
                 if not self._wallet.wallet.lnworker.channels:
-                    self.validationWarning.emit('no_channels',_('Detected valid Lightning invoice, but there are no open channels'))
+                    if bip21 and 'address' in bip21:
+                        self._logger.debug('flow where invoice has both LN and onchain, we have LN enabled but no channels')
+                        self._validateRecipient_bip21_onchain(bip21)
+                    else:
+                        self.validationWarning.emit('no_channels',_('Detected valid Lightning invoice, but there are no open channels'))
                 else:
+                    self.setValidLightningInvoice(lninvoice)
                     self.validationSuccess.emit()
         else:
             self._logger.debug('flow without LN but having bip21 uri')