fix(babel): Handle ignored files in babel v7

[danez]

Summary

In babel v7 babel-core.transform() returns null if a file is ignored.
This change makes babel-jest not throw a TypeError in this case.

Test plan

Tested by modifying local installed babel-jest in babel, while trying to get jest running in babel itself.

[codecov-io]

Codecov Report

Merging #4393 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4393   +/-   ##
=======================================
  Coverage   56.17%   56.17%           
=======================================
  Files         191      191           
  Lines        6424     6424           
  Branches        6        6           
=======================================
  Hits         3609     3609           
  Misses       2812     2812           
  Partials        3        3

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[cpojer]

Thanks for submitting a fix :)

[github-actions]

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