Entity search using notIn, <> or doesNotContain operators doesn't return records with null

[vyacheslav-pushkin]

{"conditions":[{"property":"stringAttr","operator":"notIn","value":["1ubupyH2Zs"]}]}
{"conditions":[{"property":"stringAttr","operator":"<>","value":"1ubupyH2Zs"}]}
{"conditions":[{"property":"stringAttr","operator":"doesNotContain","value":"1ubup"}]}

Neither of these will return records where stringAttr is null.

Generic UI returns such records when using notIn operator, but not when using <> or doesNotContain.