Fix jest crashing when potentialError is null

[natealcedo]

Summary

This PR fixes the scenario where a test returns a promise which rejects to null.

Fixes #6896
Closes #6897

Test plan

An end to end test has been added to test for this scenario.
Note that the test assertion has two cases due to jest-jasmine and jest-circus returning different outputs for this case.

Edit:
Acknowledging @Pajn here for the work he has done in #6897.
Just noting that #6897 should also be closed when this PR is accepted.

[codecov-io]

Codecov Report

Merging #7049 into master will not change coverage.
The diff coverage is 0%.

@@           Coverage Diff           @@
##           master    #7049   +/-   ##
=======================================
  Coverage   66.86%   66.86%           
=======================================
  Files         250      250           
  Lines       10460    10460           
  Branches        3        4    +1     
=======================================
  Hits         6994     6994           
  Misses       3465     3465           
  Partials        1        1

Impacted Files	Coverage Δ
packages/jest-jasmine2/src/is_error.js	0% <0%> (ø)	⬆️

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[SimenB]

This is great, thank you!

[github-actions]

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