fix: handle null being passed to createTransformer

[yinm]

Summary

I took a regression bug in babel-jest.
The regression bug occurs since bb720d2#diff-ad0be0b5e1f4711c408a42bdbea512bcR43-R57 .

fixes #9926

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[codecov-io]

Codecov Report

Merging #9955 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #9955      +/-   ##
==========================================
- Coverage   64.08%   64.06%   -0.02%     
==========================================
  Files         293      293              
  Lines       12457    12457              
  Branches     3069     3067       -2     
==========================================
- Hits         7983     7981       -2     
- Misses       3832     3833       +1     
- Partials      642      643       +1     

Impacted Files	Coverage Δ
packages/babel-jest/src/index.ts	51.42% <100.00%> (ø)
packages/expect/src/utils.ts	94.96% <0.00%> (-1.26%)	⬇️

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[SimenB]

Thanks @yinm, this is great! I've only got a nit for the changelog, code and test LGTM 👍

[yinm]

Thank you for the review 🙏
I incorporated your suggestion, so please check it.

[SimenB]

Thanks @yinm!

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