McBreadcrumbsConfig.postProcess nullable

[XavierDupessey]

Hello,

I think the definition of McBreadcrumbsConfig.postProcess should be IPostProcessFunc | null | undefined to be able to reset it without Typescript compilation error.

A workaround is to assign an empty function but it's a shame: the code actually works with null or undefined.

I can make a PR if you agree but I think it will be longer to review it rather than changing it directly.