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

[DmytroAlipov]

This is a backport from the master branch.

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 (ba9fb37) to head (bd25b6b).
Report is 1 commits behind head on open-release/quince.master.

Additional details and impacted files

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##           open-release/quince.master     #386   +/-   ##
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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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[DmytroAlipov]

Hi @arbrandes
Could you please merge this MR so that these changes are included in Quince.3?

[cmltaWt0]

@DmytroAlipov I think it's good to test and merge this since the master pr has been merged.
I'm going to build it and test on the test Quince env.

[cmltaWt0]

Verified on test Env - looks great.
Merging.

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