Add null QVariant constructor

[direc85]

QVariant::default() yields an undefined value in QML, but in many places a null value is more suited. Add QVariant::null() function to achieve that. The test added help distinguishing this.

I would have used QVariant(nullptr) instead, but that doesn't seem to work well with the type system in Qt5. The QJsonValue().toVariant() is the next best option - and makes sense, since a JSON can also have an explicit null value.

[ogoffart]

Sorry for not having answer to this PR before.

I don't think it makes sense to construct a QVariant with a JSValue from this API. If you want a JSValue, IMHO you should construct a JSValue and convert that to a QVariant.

[direc85]

I have no idea how the test was a merge conflict, but it's now rebased to master.

I agree, it doesn't really make sense to construct a null QVariant from QJsonValue, but that was the most elegant way I was able to find for Qt 5, since QVariant::fromValue(nullptr) doesn't work until Qt 6. I mitigated this by making it pick the choice compile time, that's something at least.

But now that I think it twice, and browsed the codebase more, it makes more sense to use QJsonValue.toVariant() as it's already available. I think I'll explore that for a bit to see that's usable already.