Qt tx dialog: handle "empty" locktime field

fix https://github.com/spesmilo/electrum/issues/5486#issuecomment-696276020

electrum/gui/qt/transaction_dialog.py

@@ -875,6 +875,8 @@ class PreviewTxDialog(BaseTxDialog, TxEditor):
         assert self.tx
         self.finalized = True
         self.tx.set_rbf(self.rbf_cb.isChecked())
+        locktime = self.locktime_e.get_locktime()
+        if locktime is not None:
             self.tx.locktime = self.locktime_e.get_locktime()
         for widget in [self.fee_slider, self.fee_combo, self.feecontrol_fields, self.rbf_cb,
                        self.locktime_setter_widget, self.locktime_e]:

electrum/transaction.py

@@ -546,7 +546,8 @@ class Transaction:
         return self._locktime
 
     @locktime.setter
-    def locktime(self, value):
+    def locktime(self, value: int):
+        assert isinstance(value, int), f"locktime must be int, not {value!r}"
         self._locktime = value
         self.invalidate_ser_cache()