dppx missing != unknown

[stefano-xy]

There should be 3 options for the dppx column: a given value, an unknown value (the device has a dppx but it's not present in the db) and not applicable (the device doesn't have a dppx).

The 3rd case is for devices that have no software (in general a browser). For example desktop monitors. These are a minority, but still ...

I suggest the following change of semantic in the JSON file:

Key -> Value (1st, a given value, show it)
Key missing (2nd, the value is not known, show '?')
Key -> Null (3rd, not applicable, show '-' or 'n/a')