What purpose does makeEmptyFunction serve in fbjs?

I was looking through the React source and stumbled across a requirement with var emptyFunction = require('fbjs/lib/emptyFunction');.

I looked at this function and was confused by what it does.

Here is the function

function makeEmptyFunction<T>(arg: T): (...args: Array<any>) => T {
  return function() {
    return arg;
  };
}

const emptyFunction: (...args: Array<any>) => void = function() {};

In the comments, they give the following explanation which I was confused by:

This function accepts and discards inputs; it has no side effects. This is primarily useful idiomatically for overridable function endpoints which always need to be callable, since JS lacks a null-call idiom ala Cocoa

I have never come across null call idiom and was hoping someone could clarify what this means and explain the purpose of this function in less technical language.

Hopefully this question will not get looked down on because it isn't exactly code related. Maybe it belongs somewhere else, if so I'm sorry.