Fix creation of missing submissions

[damianhxy]

Description

• Only run autocomplete code on create submissions page when CUD is nil.

Motivation and Context

As reported in INC1029200, an internal server error occurs when one attempts to fill in blank submissions for missing submissions.

This is because the autocomplete js code attempts to run even when CUD is set (so there's no need to autocomplete anything). However, the required data is not initialized in such a case, so we attempt to iterate over nil.

Fill in Empty Submissions feature

Server only initializes autocomplete data when CUD is not nil

To remedy this, don't run autocomplete js code unless there is a need to.

How Has This Been Tested?

• On an assessment with missing submissions, attempt to fill in empty submissions

Before changes

After changes

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have run rubocop for style check. If you haven't, run overcommit --install && overcommit --sign to use pre-commit hook for linting
• My change requires a change to the documentation, which is located at Autolab Docs
• I have updated the documentation accordingly, included in this PR

[michellexliu]

lgtm!