feat: Allow a ref to be undefined in the types

[skvale]

Thanks for the library!

Summary

I use react-merge-refs for optional refs that may or may not be passed into a component:

const mergedRef = mergeRefs([inputProps?.ref, localRef]);

This functionally works, but it shows a type error

Type 'undefined' is not assignable to type 'LegacyRef | MutableRefObject'.

Test plan

The included test checks the types are correct as well as the behavior when passing null or undefined to mergeRefs

[gregberge]

Thanks for this addition. Could you please allow null value and run npm run format to fix the formatting?

[skvale]

How does b877f71 look?

[gregberge]

@skvale released!