I get back no results. I have this problem at multiple points in my codebase, and it blew when I tried to migrate my existing sphinx index to manticore.
NULL is only supported for JSON properties.
Manticore and Sphinx2 (no idea about Sphinx3) never had proper NULL support for regular attributes as empty strings and NULL (coming from a DB source) are stored as ‘’ (same goes for float/attributes -> NULLs from DB are stored as 0/0.0) and I think IS [NOT] NULL worked incorrectly in the past as it was matching NULL=’’.