I did not yet think about this one in deep yet not in all search engines something like null does exists, example some even don't allow to send null and require to send nothing instead.

I'm not even yet sure if I for example indexing multi values like in the test example "tags": ["Tag A", "Tag B"] and should be even possible with a null via "tags": ["Tag A", null, "Tag C"], its sure more technical not a real world example.

But maybe something like a IsEmptyCondition make sense. This would be keep in mind that "title": "" and "title": null at the end would be the same. And Empty would on multi values only match when it is [].