I'm also slightly confused now, but I think there are two scenarios which we currently don't handle:

1. Columns with empty string values (not the same as null columns) - currently we don't allow those columns in the result set
2. Columns with `null` as a value (no column value) - we also don't allow those in the result set

Allowing 1 should be straight forward, but I would need dig in for 2.