feat: check for server before creating treewalker from document

[SaadBazaz]

Hey,

I've been trying to get Material Components to work in NextJS SSG. All other components work fine except for Dialog, which throws the error document is not defined.

Upon inspection, the reason is because Dialog uses document.treeWalker upon initialization. I've added a check to initialize it as an empty object typecasted to TreeWalker, so that it works fine in SSG.

Tested, works ok. However, I'd love to know if there's a better way, instead of typecasting.

[asyncLiz]

Good catch! One small change request to have stricter types

[asyncLiz]

Thank you!