The repository contains mix:title nodes, in one of these nodes the jcr:description property is null.
root
document1 (mix:title) jcr:title="Star wars" jcr:description="Dart rules!!"
document2 (mix:title) jcr:title="Prison break" jcr:description="Run, Forest, run ))"
document3 (mix:title) jcr:title="Titanic" // The description property does not exist. This is the node we wish to find.