Describe the bug
A clear and concise description of what the bug is.

To Reproduce
Steps to reproduce the behavior:

const m = moment(null);
const json = moment.toJSON();

assert(typeof json === 'string'); // fails

Expected behavior

The TS type definitions state that the moment() function accepts null as the input, which it does, but it produces a monent instance with an "Invalid date".
The TS type definitions also state that Moment.toJSON() returns string, but if the date is invalid, it will return null.

TS types must be trusted, that's kind of the point. I had correctly typed data (my input was typed string | undefined | null), and called moment(input).toJSON(), expecting to get a string back, but got null, which made my application crash.

Desktop (please complete the following information):

• OS: Mac, Node.JS 18.x

Please run the following code in your environment and include the output:

> console.log((new Date()).toString())
Thu Jan 11 2024 12:12:12 GMT+0100 (Central European Standard Time)
undefined
> console.log((new Date()).toLocaleString())
1/11/2024, 12:12:12 PM
undefined
> console.log((new Date()).getTimezoneOffset())
-60
undefined
> console.log(navigator.userAgent)
Uncaught ReferenceError: navigator is not defined
> console.log(moment.version)
2.29.4

Additional context