For nullable select fields, it should be possible to filter on the empty value, while it is not:
(I suppose there should be a "No species" option too in that menu?)

as a consequence some records seem to be missing, ex in the below recording we probably expect to see pets without species because they aren't cats
https://github.com/user-attachments/assets/33bbdb95-6122-48b1-bbb2-f84c627e1003