[@types/luxon] Add possible null interval intersection result

[kmontag]

Please fill in this template.

• Use a meaningful title for the pull request. Include the name of the package modified.
• Test the change in your own code. (Compile and run.)
• Add or edit tests to reflect the change. (Run with npm test.)
• Follow the advice from the readme.
• Avoid common mistakes.
• Run npm run lint package-name (or tsc if no tslint.json is present).

Select one of these and delete the others:

If changing an existing definition:

• Provide a URL to documentation or source code which provides context for the suggested changes: https://github.com/moment/luxon/blob/4c4f9021362432e1406a3a147a916122b676affb/src/interval.js#L405
• Increase the version number in the header if appropriate.
• If you are making substantial changes, consider adding a tslint.json containing { "extends": "dtslint/dt.json" }.

a.intersection(b) for Intervals a and b will return null if a and b do not overlap. This adds null to the method's possible return types.

[typescript-bot]

@kmontag Thank you for submitting this PR!

🔔 @colbydehart @FourwingsY @jsiebern @mastermatt @pietrovismara @dawnmist - please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

If no reviewer appears after a week, a DefinitelyTyped maintainer will review the PR instead.

[mastermatt]

Thanks

[typescript-bot]

A definition owner has approved this PR ⭐️. A maintainer will merge this PR shortly. If it shouldn't be merged yet, please leave a comment saying so and we'll wait. Thank you for your contribution to DefinitelyTyped!

[typescript-bot]

I just published @types/luxon@1.11.1 to npm.