fix compatibility with @babel/core option check when inputSourceMap null

[Delagen]

Please Read the CONTRIBUTING Guidelines
In particular the portion on Commit Message Formatting

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior? (You can also link to an open issue here)
Assertion in @babel/core package when file come from another loader without sourceMap
In my case val-loader with only code option.
https://github.com/babel/babel/blob/master/packages/babel-core/src/config/option-assertions.js#L71

What is the new behavior?
No errors on compilation.

Does this PR introduce a breaking change?

• Yes
• No

Other information:

[danez]

Sorry for the late reply. So this fails if inputSourceMap is null? Do I understand this correctly?

Can you please add a comment describing why we need to default to null, because I already see that in a year someone might remove it because it is not common to default to undefined. :)

[Delagen]

@danez Yes. Because babel-core only check for undefined, and throw exception if it null. I'll test it with latest versions and comment if issue still present

[Delagen]

@danez Seems latest builds of @babel/core does not affected, so I close it for now.

[Delagen]

Again get error Module build failed: Error: .inputSourceMap must be a boolean, object, or undefined

[danez]

Thanks

[Delagen]

@danez https://github.com/babel/babel/blob/493996e02a3e22ae4e89443f4df106ea6bc3774c/packages/babel-core/src/config/validation/option-assertions.js So it must be omited from params or undefined

[danez]

Yeah i noticed that, not sure why it cannot be null. @loganfsmyth

This fix fix should work for now.