Fixes AttributeError when trying to get `num_sats_can_receive()` in
wallets without lnworker. Fixes https://github.com/spesmilo/electrum/issues/8100#issuecomment-3294556043.
```
Traceback (most recent call last):
File "/usr/lib/python3.13/site-packages/electrum/gui/qt/request_list.py", line 103, in selection_changed
self.receive_tab.update_current_request()
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^
File "/usr/lib/python3.13/site-packages/electrum/gui/qt/receive_tab.py", line 237, in update_current_request
help_texts = self.wallet.get_help_texts_for_receive_request(req)
File "/usr/lib/python3.13/site-packages/electrum/wallet.py", line 3446, in get_help_texts_for_receive_request
can_receive = self.lnworker.num_sats_can_receive()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'num_sats_can_receive'
```
b6a4ebecda