Convert: return nil when detector is empty

When PotentialNamespace is used detector is empty and
DetectorFromDatabaseModel() should return nil instead of empty object.

api/v3/clairpb/convert.go

@@ -153,6 +153,9 @@ func NamespacedFeatureFromDatabaseModel(feature database.AncestryFeature) *Featu
 
 // DetectorFromDatabaseModel converts database detector to api detector.
 func DetectorFromDatabaseModel(detector database.Detector) *Detector {
+	if !detector.Valid() {
+		return nil
+	}
 	return &Detector{
 		Name:    detector.Name,
 		Version: detector.Version,