lnworker: fix get_onchain_history if running with --offline

```
Traceback (most recent call last):
  File "...\electrum\electrum\gui\qt\main_window.py", line 898, in timer_actions
    self.update_wallet()
  File "...\electrum\electrum\gui\qt\main_window.py", line 1040, in update_wallet
    self.update_tabs()
  File "...\electrum\electrum\gui\qt\main_window.py", line 1047, in update_tabs
    self.history_model.refresh('update_tabs')
  File "...\electrum\electrum\util.py", line 439, in <lambda>
    return lambda *args, **kw_args: do_profile(args, kw_args)
  File "...\electrum\electrum\util.py", line 435, in do_profile
    o = func(*args, **kw_args)
  File "...\electrum\electrum\gui\qt\history_list.py", line 275, in refresh
    transactions = wallet.get_full_history(
  File "...\electrum\electrum\util.py", line 439, in <lambda>
    return lambda *args, **kw_args: do_profile(args, kw_args)
  File "...\electrum\electrum\util.py", line 435, in do_profile
    o = func(*args, **kw_args)
  File "...\electrum\electrum\wallet.py", line 947, in get_full_history
    lnworker_history = self.lnworker.get_onchain_history() if self.lnworker and include_lightning else {}
  File "...\electrum\electrum\lnworker.py", line 911, in get_onchain_history
    tx_height = self.lnwatcher.get_tx_height(swap.funding_txid)
AttributeError: 'NoneType' object has no attribute 'get_tx_height'
```

electrum/lnworker.py

@@ -908,9 +908,10 @@ class LNWallet(LNWorker):
                 amount_msat = 0
             label = 'Reverse swap' if swap.is_reverse else 'Forward swap'
             delta = current_height - swap.locktime
-            tx_height = self.lnwatcher.get_tx_height(swap.funding_txid)
-            if swap.is_reverse and tx_height.height <=0:
-                label += ' (%s)' % _('waiting for funding tx confirmation')
+            if self.lnwatcher:
+                tx_height = self.lnwatcher.get_tx_height(swap.funding_txid)
+                if swap.is_reverse and tx_height.height <= 0:
+                    label += ' (%s)' % _('waiting for funding tx confirmation')
             if not swap.is_reverse and not swap.is_redeemed and swap.spending_txid is None and delta < 0:
                 label += f' (refundable in {-delta} blocks)' # fixme: only if unspent
             out[txid] = {