readFileSync `options` can be null

[cjbarth]

export function readFileSync(filename: string, options: ReadFileOptions): Buffer;
and
export function readFile(filename: string, options: ReadFileOptions, callback: (err: NodeJS.ErrnoException | null, data: Buffer) => void): void;

should be:

export function readFileSync(filename: string, options: ReadFileOptions | null): Buffer;
and
export function readFile(filename: string, options: ReadFileOptions | null, callback: (err: NodeJS.ErrnoException | null, data: Buffer) => void): void;

I can create a PR if desired.

[blakeembrey]

Sure, I think that's reasonable. Currently it supports undefined instead of null. Handling from node.js core: https://github.com/nodejs/node/blob/0bdd39f729a9c26a88cda964696b97bdaa2592d4/lib/fs.js#L40-L43.

Edit: I'd augment every fs method with this too.

[blakeembrey]

Thanks! Closed with your PR: #47.