PagedList: Replace nullable types with an Option utility.

[az-raven]

Currently, we use T where T: class in PagedList as well as the null value as an empty slot in the list. This is a bit of a hack, and it would be better to use an Option type or similar to represent the null value. This would also allow us to use value types in PagedList.

FIXME for reference:

SharedUtils/PagedList.cs

Lines 6 to 7 in 96e4e95

	// FIXME: This should be converted to use TaggedUnion<A, B> instead of
	// using nulls to represent empty items.