feat(material/table): accept undefined sort and paginator

[rslawik]

Extend types of the sort and paginator setter to include undefined to avoid ?? null when using singals (e.g. viewChild).

[rslawik]

Based on internal testing: I reduced the scope of this change to only accept undefined in the setter without coercing it to null because the initial values of this._paginator and this._sort are undefined exposed as null. This avoids build and test breakages.

We can accept the casts or consider making a breaking change as a follow up to either:

1. Initialize this._sort: MatSort | null = null and coerce undefined to null in the setter. There are a handful targets to fix for that.
2. Expose undefined in get sort(): MatSort | null | undefined and in this._sort. There are a few generic components wrapping the data source to fix.
3. Replace null with undefined. This should be equivalent to 2 in effort.

Same for the paginator. These are fairly tractable to migrate incrementally, so I can help. But the decision which one should be made by the owner.