[Popper] Update TypeScript definitions

[Slessi]

• Add missing transition prop
• Add null as option to anchorEl (strictNullChecks will complain that null is not valid)
• Add childProps def for children

[oliviertassinari]

Thanks for cleaning that up.

[oliviertassinari]

How is ? not enough?

[Slessi]

With strictNullChecks or strict mode turned on, ? only allows for passing undefined, but passing null does work too.

In reality, any developer using strict should be capable of figuring out that they can just use undefined but documentation example was using null so dunno thought I'd add it.

Not really important.

[oliviertassinari]

@Slessi Thank you