a nodestyle json transform problem

[Larry-Zheng]

I am trying to run it on lower version qt when I found a bug in NodeGraphicsObject::NodeGraphicsObject.
QJsonObject nodeStyleJson = _graphModel.nodeData(_nodeId, NodeRole::Style).toJsonObject();
Actually， what nodeData return is a variant map， please construct the json object explicitly.
auto nodeStyleJson = QJsonObject::fromVariantMap(_graphModel.nodeData(_nodeId, NodeRole::Style).toMap());

[Llcoolsouder]

Can you open a PR with the changes you'd like to see?