use an empty list instead of [None] if revision.user doesn't exists

[yvess]

this fixes #7879

Please check the following:

• Do the tests still pass?¹
• Does the code comply with the style guide?
  • Run make lint from the Wagtail root.
• For Python changes: Have you added tests to cover the new/fixed behaviour?
• For front-end changes: Did you test on all of Wagtail’s supported environments?²
  • Please list the exact browser and operating system versions you tested:
  • Please list which assistive technologies ³ you tested:
• For new features: Has the documentation been updated accordingly?

Please describe additional details for testing this change.

As described in #7879:

Steps to Reproduce

• Enter the Django shell and import the page model.
• Execute page.save_revision(submitted_for_moderation=True)
• See your revision in the moderation queue (as a moderator user), and attempt to accept the revision

Now it passes, before it showed a 500 error

Footnotes

1. Development Testing ↩

2. Browser and device support ↩

3. Accessibility Target ↩

[squash-labs]

Manage this branch in Squash

Test this branch here: https://yvessfix7879-revision-without-1l311.squash.io

[gasman]

Thanks @yvess!
We should definitely add a test for this case in test_moderation.py, to verify that this has been fixed and ensure that the bug doesn't get re-introduced in future code changes.

Essentially we want a test case that performs the steps described in #7879, and I think we can get there by copying the setUp and test_approve_moderation_view functions into a new class - named something like TestApproveRejectModerationWithoutUser - and removing the user argument from the line self.page.save_revision(user=self.submitter, submitted_for_moderation=True). When I try that locally on an unpatched copy of Wagtail, the test then fails with AttributeError: 'NoneType' object has no attribute 'is_active' which I think is the error we're expecting.

[gasman]

Looks good to me - thanks!

[gasman]

Merged in e2e356d.