[styles] Fix createStyles for TypeScript v3.5

[merceyz]

• I have followed (at least) the PR section of the contributing guide.

For TypeScript v3.5 P has to extend {} instead of object
See #15942
and microsoft/TypeScript#31735

Closes #15942

[mui-pr-bot]

No bundle size changes comparing b3cace6...3a1a263

Generated by 🚫 dangerJS against 3a1a263

[eps1lon]

Could you link to the part in the devblog that talks about object vs {}?

[merceyz]

It's the first item in breaking changes, can't link to it but search for

As a workaround, you can add an explicit constraint of {} to a type parameter to get the old behavior

[eps1lon]

I don't understand how these breaking changes relate to this change. The breaking change is talking about unconstrained generic arguments. The generic arguments in createStyles are constrained however.

This sounds like either a bug in TS or an undocumented breaking change. I'll look through the issues and see if I can find a minimal example for the changed behavior.

[eps1lon]

@merceyz Thanks for looking into this!