Node fails to commission with error in region controller re: '50-maas-01-commissioning' failed during post-processing

When Commissioning a new node (dell r620) it fails without noting an error in the MaaS UI on the `50-maas-01-commissioning` stage. An exception is identified in the region controller's regiond.log:

```
2021-08-09 15:39:11 metadataserver.api: [critical] close-fly.fdcservers.maas(68fepw): commissioning script '50-maas-01-commissioning' failed during post-processing.
 Traceback (most recent call last):
   File "/usr/lib/python3/dist-packages/metadataserver/api.py", line 820, in signal
     target_status = process(node, request, status)
   File "/usr/lib/python3/dist-packages/metadataserver/api.py", line 641, in _process_commissioning
     self._store_results(
   File "/usr/lib/python3/dist-packages/metadataserver/api.py", line 529, in _store_results
     script_result.store_result(
   File "/usr/lib/python3/dist-packages/metadataserver/models/scriptresult.py", line 384, in store_result
     signal_status = try_or_log_event(
 --- <exception caught here> ---
   File "/usr/lib/python3/dist-packages/metadataserver/api.py", line 447, in try_or_log_event
     func(*args, **kwargs)
   File "/usr/lib/python3/dist-packages/metadataserver/builtin_scripts/hooks.py", line 919, in process_lxd_results
     _process_lxd_resources(node, data)
   File "/usr/lib/python3/dist-packages/metadataserver/builtin_scripts/hooks.py", line 591, in _process_lxd_resources
     update_node_devices(
   File "/usr/lib/python3/dist-packages/metadataserver/builtin_scripts/hooks.py", line 549, in update_node_devices
     _process_usb_devices(add_func, data)
   File "/usr/lib/python3/dist-packages/metadataserver/builtin_scripts/hooks.py", line 472, in _process_usb_devices
     [
 builtins.TypeError: 'NoneType' object is not iterable
```

Combined log output is attached.