qt tx dialog: fix "preview" for unsigned pre-segwit tx

```
629.08 | E | gui.qt.exception_window.Exception_Hook | exception caught by crash reporter
Traceback (most recent call last):
  File ".../electrum/electrum/gui/qt/invoice_list.py", line 170, in <lambda>
    menu.addAction(_("Pay") + "...", lambda: self.send_tab.do_pay_invoice(invoice))
  File ".../electrum/electrum/gui/qt/send_tab.py", line 573, in do_pay_invoice
    self.pay_onchain_dialog(self.window.get_coins(), invoice.outputs)
  File ".../electrum/electrum/gui/qt/send_tab.py", line 251, in pay_onchain_dialog
    self.window.show_transaction(tx)
  File ".../electrum/electrum/gui/qt/main_window.py", line 1074, in show_transaction
    show_transaction(tx, parent=self, desc=tx_desc)
  File ".../electrum/electrum/gui/qt/transaction_dialog.py", line 351, in show_transaction
    d = TxDialog(tx, parent=parent, desc=desc, prompt_if_unsaved=prompt_if_unsaved)
  File ".../electrum/electrum/gui/qt/transaction_dialog.py", line 450, in __init__
    self.set_title()
  File ".../electrum/electrum/gui/qt/transaction_dialog.py", line 858, in set_title
    self.setWindowTitle(_("Transaction") + ' ' + self.tx.txid())
TypeError: can only concatenate str (not "NoneType") to str
```

electrum/gui/qt/transaction_dialog.py

@@ -855,7 +855,8 @@ class TxDialog(QDialog, MessageBoxMixin):
         self.locktime_final_label.setVisible(True)
 
     def set_title(self):
-        self.setWindowTitle(_("Transaction") + ' ' + self.tx.txid())
+        txid = self.tx.txid() or "<no txid yet>"
+        self.setWindowTitle(_("Transaction") + ' ' + txid)
 
     def can_finalize(self) -> bool:
         return False