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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 (symbol meaning) served by clicking on 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.

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.

Seeing that ∅ is your choice for an empty value, for non-empty value 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. Luckily, as the dropdown has text to explain 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 can always click on the dropdown to see what it means.

Given than, you may just as well make up something slightly more memorable, like ○ for empty, and ● for not-empty.

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 (symbol meaning) served by clicking on 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.

If ∅ is your choice for an empty value, than for non-empty value you can just prepend the negation symbol: ¬∅, or !∅.

As far as UX goes, this will only be instantly clear to people who are familiar with logic symbols. Luckily, as the dropdown has text to explain 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 can always click on the dropdown to see what it means.

Given than, you may just as well make up something slightly more memorable, like ○ for empty, and ● for not-empty.

Seeing that ∅ is your choice for an empty value, for non-empty value 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. Luckily, as the dropdown has text to explain 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 can always click on the dropdown to see what it means.

Given than, you may just as well make up something slightly more memorable, like ○ for empty, and ● for not-empty.