fix: "An unexpected error occurred" after second opening any grade

[DmytroAlipov]

Hi! I found a bug.

1. Open Instructor -> Student admin -> View gradebook

2. Click on some grade
   

3. Set new grade or just click Cancel

4. Open same grade or another one
   

This happens because doneViewingAssignment() executes when the modal window closes. The redux state is being cleaned, namely, gradeOverrideHistoryResults is removed.
Because of this, when we access this element again, we do not get the expected iterable object (list), but we get an undefined.
Previously, this was eliminated by the default value.
My suggestion is to also make the value default [] if undefined.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.98%. Comparing base (6450d86) to head (4e9270a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #387   +/-   ##
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  Coverage   94.98%   94.98%           
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  Files         140      140           
  Lines        1356     1356           
  Branches      264      265    +1     
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  Hits         1288     1288           
  Misses         60       60           
  Partials        8        8           

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[mphilbrick211]

Hi @DmytroAlipov! I believe Aurora is no longer maintaining this repo, so I'm going to work on finding another reviewer for these. Thanks!

[arbrandes]

I have a strong suspicion that this will solve #384. Thanks, I'll review ASAP!

[cmltaWt0]

@arbrandes Could you take a look at this?

[arbrandes]

Looks safe enough. Approved!

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