qt send: use bolt11 fallback addr even if LN is disabled

Given a wallet with LN disabled,
and a bolt11 invoice (or a bip21 uri that only contains bolt11 but lacks a top-level address),
if the bolt11 invoice includes a fallback address,
we would previously just error "Lightning is disabled".

Now we offer the user to pay on-chain using the fallback address.

closes https://github.com/spesmilo/electrum/issues/8047

electrum/invoices.py

@@ -136,6 +136,12 @@ class Invoice(StoredObject):
             outputs = self.outputs
         return outputs
 
+    def can_be_paid_onchain(self) -> bool:
+        if self.is_lightning():
+            return bool(self._lnaddr.get_fallback_address())
+        else:
+            return True
+
     def get_expiration_date(self):
         # 0 means never
         return self.exp + self.time if self.exp else 0