Fix bug to allow users to unpair sameAs value in Course Modal

[natalynyu]

This fixes a bug in the Course modal that was preventing users from being able to save the saveAs value as null when the course previously had a non null value.

Types of changes

• [x] 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:

• [x] I have run eslint on the code
• [x] I have added JSDoc for all of my code (where applicable)
• [x] I have added tests to cover my changes.

Priority:

• [x] Normal
• [ ] High

Related Issues:

Fixes #650

[codecov[bot]]

Codecov Report

Merging #666 (934262e) into develop (b293fcf) will increase coverage by 0.00%. The diff coverage is 100.00%.

:exclamation: Current head 934262e differs from pull request most recent head 1f092ea. Consider uploading reports for the commit 1f092ea to get more accurate results

@@           Coverage Diff            @@
##           develop     #666   +/-   ##
========================================
  Coverage    94.19%   94.20%           
========================================
  Files          195      196    +1     
  Lines         4602     4607    +5     
  Branches       567      567           
========================================
+ Hits          4335     4340    +5     
  Misses         149      149           
  Partials       118      118           

Impacted Files	Coverage Δ
src/client/components/App.tsx	96.77% <100.00%> (-0.11%)	:arrow_down:
src/client/components/layout/CustomFooter.tsx	100.00% <100.00%> (ø)
src/client/components/layout/index.ts	100.00% <100.00%> (ø)
src/server/course/course.controller.ts	94.33% <100.00%> (+0.10%)	:arrow_up:

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