Seeing that ∅
is your choice for an empty value, for non-empty one you can just prepend it with the negation symbol: ¬∅
, or !∅
.
As far as UX goes, this will only be instantly clear to people who are familiar with logic symbols (or most developers in the case of !
). Luckily, as each dropdown item has text to elucidate what each symbol means, and as it may be fair to assume that most users will be familiar with most other symbols, having a few cryptic symbols is not such a big deal - users will be aware of the contextual help served by the dropdown.
Given that, you may just as well make up something slightly more memorable, like ○
for empty, and ●
for non-empty. It may not be based on logic notation, but will work all the same.