lnchannel: get_capacity() should not raise when running with --offline
lnchannel: get_capacity() should not raise when running with --offline

```
E | gui.qt.exception_window.Exception_Hook | exception caught by crash reporter
Traceback (most recent call last):
  File "...\electrum\electrum\gui\qt\channels_list.py", line 295, in do_update_rows
    field_map = self.format_fields(chan)
  File "...\electrum\electrum\gui\qt\channels_list.py", line 99, in format_fields
    capacity_str = self.parent.format_amount(chan.get_capacity(), whitespaces=True)
  File "...\electrum\electrum\lnchannel.py", line 481, in get_capacity
    return self.lnworker.lnwatcher.get_tx_delta(self.funding_outpoint.txid, self.cb.funding_address)
AttributeError: 'NoneType' object has no attribute 'get_tx_delta'
```

spesmilo/electrum,pooler/electrum-ltc,spesmilo/electrum,spesmilo/electrum,pooler/electrum-ltc,pooler/electrum-ltc,spesmilo/electrum,pooler/electrum-ltc