wallet.sign_transaction: return tx if signed by swap manager

This fixes bumping swap fee in the qml GUI, because it expects
the value returned by this method to be None if the transaction
could not be signed.

electrum/wallet.py

@@ -2318,6 +2318,7 @@ class Abstract_Wallet(ABC, Logger, EventListener):
         self._add_txinout_derivation_info(txout, address, only_der_suffix=only_der_suffix)
 
     def sign_transaction(self, tx: Transaction, password) -> Optional[PartialTransaction]:
+        """ returns tx if successful else None """
         if self.is_watching_only():
             return
         if not isinstance(tx, PartialTransaction):
@@ -2326,7 +2327,7 @@ class Abstract_Wallet(ABC, Logger, EventListener):
         swap = self.get_swap_by_claim_tx(tx)
         if swap:
             self.lnworker.swap_manager.sign_tx(tx, swap)
-            return
+            return tx
         # add info to a temporary tx copy; including xpubs
         # and full derivation paths as hw keystores might want them
         tmp_tx = copy.deepcopy(tx)