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'
```

electrum/lnchannel.py

@@ -407,6 +407,11 @@ class AbstractChannel(Logger, ABC):
         """Returns our node ID."""
         pass
 
+    @abstractmethod
+    def get_capacity(self) -> Optional[int]:
+        """Returns channel capacity in satoshis, or None if unknown."""
+        pass
+
 
 class ChannelBackup(AbstractChannel):
     """
@@ -478,7 +483,10 @@ class ChannelBackup(AbstractChannel):
         return self.is_imported or self.is_redeemed()
 
     def get_capacity(self):
-        return self.lnworker.lnwatcher.get_tx_delta(self.funding_outpoint.txid, self.cb.funding_address)
+        lnwatcher = self.lnworker.lnwatcher
+        if lnwatcher:
+            return lnwatcher.get_tx_delta(self.funding_outpoint.txid, self.cb.funding_address)
+        return None
 
     def is_backup(self):
         return True