Search filters is not taken into account on null field in local extension repository

{{groovy}}
import com.xpn.xwiki.web.Utils;
import org.xwiki.extension.repository.search.ExtensionQuery;

def localExtensionRepository = Utils.getComponent(org.xwiki.extension.repository.LocalExtensionRepository.class);

def query = new ExtensionQuery();

println "= All extensions ="
for (def extension : localExtensionRepository.search(query)) {
  println "* " + extension.getName();
}

println "= Flavors ="
query.addFilter("category", "flavor", ExtensionQuery.COMPARISON.EQUAL);
for (def extension : localExtensionRepository.search(query)) {
  println "* " + extension.getName();
}
{{/groovy}}