Refactor: Use nullish coalescing operator to simplify undefined checking

[Mosoc]

Typescript supports nullish coalescing operator in version 3.7

I thought it's a good idea to replace comparison between value and undefined with this feature.

From

{
   disabled: disabled != undefined ? disabled : isSubmitting,
}

to

{
    disabled: disabled ?? isSubmitting,
};

Those typescript codes would compile to javascript as:

{
    disabled: (disabled !== null && disabled !== void 0 ? disabled : isSubmitting),
};

I also upgraded prettier to support this syntax feature.

Ref:
prettier/prettier#6595

[coveralls]

Coverage increased (+1.5%) to 80.714% when pulling dc7c673 on Mosoc:master into 29848be on stackworx:master.

[cliedeman]

@Mosoc thanks! this is great