UniformValue::fromVariant doesn't handle undefined JS values

If an invalid QVariant is passed to UniformValue::fromVariant, Q_UNREACHABLE is reached and the application segfaults without notification. Ideally, the invalid QVariant should have been caught earlier and not passed to the function, but it might be an idea to pass a warning and silently handle this case (setting the value to a zero float) or at least printing a qFatal error message to provide some debug information.

This may now happen by passing an `undefined` value from QML through QML objects like `Parameter`. While passing `undefined` values from JS is a bad thing, the application shouldn't segfault and crash, but rather print an error that is visible in the application log.