lgtm.

it's a pretty uncommon use-case for the ordering field to be null

Not only that; the code also doesn't work properly in all edge cases. In particular, when there are several NULL rows (possible with nulls_distinct=True), the cursor position becomes ill-defined. The problem gets worse if those rows don't all fit on one page.

To prevent unpredictable behavior, the code would have to check that the schema doesn't allow this constellation. Alternatively, one could look into index types that work better with IS NULL conditions. However, I'm not convinced that doing all this within DRF is warranted. It's fair enough that people have what's documented ("requires that there is a unique, unchanging ordering"), and if someone has a special case, it shouldn't be a big deal to extend the class and adapt the behavior accordingly, within their app.