Can't unselect all internal device sensors, even if an external device is connected

**Describe the bug**
In the sensor settings it is not possible to unselect all internal sensors (one sensor always stays) even if an external device is connected. In comparison, the Android version lets you unselect all internal sensors as long as any external sensor of a connected device is selected.

**To Reproduce**
Steps to reproduce the behavior:
1. Go to the sensor settings
2. Add an external device
3. Select a sensor of the external device
4. Try to unselect all internal sensors

**Expected behavior**
As long as an external device is connected (and a sensor of this connected device is selected), it should be possible to unselect all internal sensors.

Start with the iOS sensor settings flow described in the reproduction steps, focusing on selection and unselection of internal and external-device sensors. Reproduce the connected-device case, compare the behavior with the Android version, and verify that all internal sensors can be unselected when a selected external sensor remains connected.

Written by the indexing model from the issue text.