witi when input_corpus is NULL

What should be the acceptable behavior of `witi(abstracts_stm)` or `create_oolong(abstracts_stm, type = "witi")`?

For old users, it should give you an oolong object with only `wi` because `input_corpus` is NULL. It is also the current behavior.

But it can be quite confusing if one asks for `wi` and `ti` but only be given an object with `wi`.

