Update: improve null detection in eqeqeq for ES6 regexes (fixes #8020)

[not-an-aardvark]

What is the purpose of this pull request? (put an "X" next to item)

[x] Bug fix (see #8020)

What changes did you make? (Give an overview)

This fixes the detection of null literals in eqeqeq, to prevent it from treating unicode/sticky regexes like null in Node 4. When parsing values that cannot be represented in the current environment (e.g. unicode regexes in Node 4), the result is a Literal node with node.value === null. The eqeqeq rule assumed that any literal node with node.value === null is a null literal.

This also adds a isNullLiteral function to ast-utils.js to prevent other rules from making the same mistake.

Is there anything you'd like reviewers to focus on?

Nothing in particular

[mention-bot]

@not-an-aardvark, thanks for your PR! By analyzing the history of the files in this pull request, we identified @mysticatea, @gyandeeps and @platinumazure to be potential reviewers.

[mysticatea]

LGTM, thank you!

[mysticatea]

I re-thought this.
node.raw property does not exist in ESTree spec.
Can we use it safely?

[not-an-aardvark]

Oh, I hadn't realized that. I suppose I can check !node.regex instead.

[eslintbot]

LGTM