TypeError: '<' not supported between instances of 'NoneType' and 'int' (for unconfirmed txs)

This is happening for unconfirmed txs:
```
[2023-01-24 11:44:15,601] ERROR in controller: Uncaught exception: '<' not supported between instances of 'NoneType' and 'int'
[2023-01-24 11:44:15,606] ERROR in controller: Traceback (most recent call last):
File "/home/kim/src/specter-desktop/.env/lib/python3.10/site-packages/flask/app.py", line 1523, in full_dispatch_request
rv = self.dispatch_request()
File "/home/kim/src/specter-desktop/.env/lib/python3.10/site-packages/flask/app.py", line 1509, in dispatch_request
return self.ensure_sync(self.view_functions[rule.endpoint])(**req.view_args)
File "/home/kim/src/specter-desktop/.env/lib/python3.10/site-packages/flask_login/utils.py", line 272, in decorated_view
return func(*args, **kwargs)
File "/home/kim/src/specter-desktop/.env/lib/python3.10/site-packages/cryptoadvance/specterext/stacktrack/controller.py", line 79, in wallets_overview
txs: list = _extract_txs(wallets)
File "/home/kim/src/specter-desktop/.env/lib/python3.10/site-packages/cryptoadvance/specterext/stacktrack/controller.py", line 124, in _extract_txs
return sorted(txs, key=lambda d: d["blockheight"], reverse=True)
TypeError: '<' not supported between instances of 'NoneType' and 'int'

[2023-01-24 11:44:15,607] INFO in _internal: 127.0.0.1 - - [24/Jan/2023 11:44:15] "GET /svc/stacktrack/wallets_overview HTTP/1.1" 500 -
```