Correction: jsguide.html - Changed null to undefined

[KumarSidharth]

This change is "Not important".

Please don't review this leaving your work behind.

Change description

Keys not defined in an Object are undefined.

const o = {width: 42};
o.maxWidth === null // false
o.maxWidth === undefined // true

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[vapier]

the example is using !=, not !==, so there is coercion involved, so I think this is written correctly following the guidance:

https://google.github.io/styleguide/jsguide.html#features-equality-checks-exceptions