SUMMARY
When we create QML object `SensorDataModel` and we set `sensors` with `enabled` set to false, the sensors MetaData is never requested so `ready` status is always "false".

STEPS TO REPRODUCE
1. Create simply applet with :
```
import QtQuick 2.9
import org.kde.ksysguard.sensors 1.0 as Sensors

Item {
  Timer {
    interval: 1000
    running: true
    onTriggered: sensorsModel.enabled = true
  }

  Sensors.SensorDataModel {
    id: sensorsModel
    enabled: false
    sensors: ["cpu/all/usage"]
    onDataChanged: (index) => console.log(index, data(index))
  }
}
```
2. See the null index and error in log.

OBSERVED RESULT

- `ready` is never true
- `data` is never updated

EXPECTED RESULT

Work as intended.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 5.27.4
(available in About System)
KDE Plasma Version: 5.27.4
KDE Frameworks Version:  5.104.0
Qt Version: 5.15.8

ADDITIONAL INFORMATION

Currently the only solution i have found is to re assign sensors when i enable "SensorDataModel". I done this with that code :
```
const sensors = sensorsModel.sensors;
sensorsModel.sensors = [];
sensorsModel.sensors = sensors;
```