Commissioning fails on ppc64 node

[Alberto Donato]

In CI, commissioning fails with 2.8 on a PPC64 node.

Logs show this traceback:

 Traceback (most recent call last):
   File "/usr/lib/python3/dist-packages/metadataserver/api.py", line 800, in signal
     target_status = process(node, request, status)
   File "/usr/lib/python3/dist-packages/metadataserver/api.py", line 622, in _process_commissioning
     node, node.current_commissioning_script_set, request, status
   File "/usr/lib/python3/dist-packages/metadataserver/api.py", line 515, in _store_results
     **args, timedout=(status == SIGNAL_STATUS.TIMEDOUT)
   File "/usr/lib/python3/dist-packages/metadataserver/models/scriptresult.py", line 391, in store_result
     exit_status=self.exit_status,
 --- <exception caught here> ---
   File "/usr/lib/python3/dist-packages/metadataserver/api.py", line 441, in try_or_log_event
     func(*args, **kwargs)
   File "/usr/lib/python3/dist-packages/metadataserver/builtin_scripts/hooks.py", line 721, in process_lxd_results
     node.save()
   File "/usr/lib/python3/dist-packages/maasserver/models/node.py", line 1954, in save
     super(Node, self).save(*args, **kwargs)
   File "/usr/lib/python3/dist-packages/maasserver/models/cleansave.py", line 216, in save
     self.validate_unique(exclude=exclude_unique_fields)
   File "/usr/lib/python3/dist-packages/django/db/models/base.py", line 1041, in validate_unique
     raise ValidationError(errors)
 django.core.exceptions.ValidationError: {'hardware_uuid': ['Node with this Hardware uuid already exists.']}

The reason for the failure is that hardware_uid is "", which makes the unique constraint fail.
It should be replaced with None if empty.