Element type parsers should return null, not default instances

[ewoutkramer]

E.g. DateTime.TryParse in the system types (namespace Element) currently has a non-nullable out parameter, which is set to default values (e.g. a DateTime with a minimal date) when the parse function returns false.

We should change this to nullables with a [NotNullWhen] attribute added to them.

This might be breaking, although the caller is not supposed to actually do something with the return value when the function returns false, so the consequences would be minor.