Provide better error message when transform returns non-string value

[strokirk]

Do you want to request a feature or report a bug?
Bug/Better error message

What is the current behavior?
Returning undefined from a transform module causes jest to report the following error:

TypeError: Cannot set property 'map' of undefined

Repl: https://repl.it/Ie2I/5

What is the expected behavior?
It would be nice to have a better error message here, or perhaps treat undefined as the empty string.

[thymikee]

Better error message would be nice here. Are you interested in providing such? :)

[github-actions]

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