Allow clearing of the child_clip_id property

Author: jonoomph

src/TrackedObjectBBox.cpp

@@ -381,7 +381,7 @@ void TrackedObjectBBox::SetJsonValue(const Json::Value root)
 		protobufDataPath = root["protobuf_data_path"].asString();
 
 	// Set the id of the child clip
-	if (!root["child_clip_id"].isNull() && root["child_clip_id"].asString() != "" && root["child_clip_id"].asString() != Id()){
+	if (!root["child_clip_id"].isNull() && root["child_clip_id"].asString() != Id()){
 		Clip* parentClip = (Clip *) ParentClip();
 		if (parentClip && root["child_clip_id"].asString() != parentClip->Id()) {
             ChildClipId(root["child_clip_id"].asString());