Filter expert: Support for not existing fields

[ghost]

It is currently not possible to filter for empty/not existing fields. As an empty value is not allowed, this value can be used for filtering, or null.

[navtej]

shouldn't sieve filter be the answer here?

[ghost]

Yeah, the sieve bot can do that. However, is it effort to configure the sieve bot acceptable for just filtering on the non-existence of a field? E.g. compare the effort to setup the filter expert with the effort to setup sieve.