Add "no-null-undefined-union" rule.

[nrathi]

PR checklist

• Addresses an existing issue: #0000
• New feature, bugfix, or enhancement
  • Includes tests
• Documentation update

CHANGELOG.md entry:

[new-rule] no-null-undefined-union

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This change is 

[palantirtech]

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[adidahiya]

almost there

[DarryQueen]

@nrathi as per this, should we stop using React.ReactNode? At least in 15.6.1, it has a type union of null | undefined.

[rafeememon]

I was pretty surprised with the effect this has on consuming React.ReactNode. In internal discussions, one proposal was to add a NonNullable wrapper, but this seems fishy because in general null is a perfectly reasonable return value for a render function. Further, in most cases, the result of the call is just passed right back into React anyway via another render. Perhaps this rule should only apply to user-land type definitions, and exclude types defined externally? I imagine this issue is not unique to React and is likely to be widespread when consuming other APIs.

[styu]

+1 to @rafeememon ’s point, it feels like the rule should be enforced on type definitions rather than on consumed types, so you wouldn’t be able to define types with the union, but could still consume types that have defined the union already. That way you can trend towards the right direction without encouraging the disabling of this rule due to external types

[adidahiya]

@styu that makes sense. I wouldn't recommend enabling this rule right away across the board. I filed #4617