fix: serialization of unfinished, unscored peer assignments

[nsprenkle]

TL;DR - Fixes issue serializing grades for unfinished, unscored peer assignments.

What changed?

We were running into the issue below when trying to get grade data for peers in new ORA experience:

AttributeError: Got AttributeError when attempting to get a value for field `assessments` on serializer `UnweightedPeerAssessmentsSerializer`. The serializer field might be named incorrectly and not match any attribute or key on the `PeerAssessmentAPI` instance. Original exception text was: 'NoneType' object has no attribute 'pk'.

It was determined that this came from serializing unscored peer workflow items that didn't have an associated assessment (unfinished assessment) based on how we are doing that lookup. Added a null check to the filter to remove these from the resopnse.

Developer Checklist

• Reviewed the release process
• Translations and JS/SASS compiled
• Bumped version number in openassessment/__init__.py and package.json

Testing Instructions

See UAT tests.

Reviewer Checklist

Collectively, these should be completed by reviewers of this PR:

• I've done a visual code review
• I've tested the new functionality

FYI: @openedx/content-aurora