Allow scoped metadata to be undefined

[davidlj95]

Current route metadta types for scoped metadata (standard, open graph, open graph profile, ...) set the proper type but don't allow undefined to be there. So in case you define a generic type that may allow some kind of scoped metadata, but may not be there in some cases, you have to do some workarounds

For instance:
https://github.com/davidlj95/chrislb/blob/4982d6aac13954f570a421cecb76ab4d04cc75f2/src/app/common/routing/make-route-metadata.ts#L38-L38

To take into account in #90

[davidlj95]

No longer an issue since refactor in #163