Fix Sensors VTS asserts for sensor type strings

Private sensors are the only sensor types that must define a type
string. For public sensors, a type string can be left empty as it will
be overridden by the sensors frameork.

Bug: 155514483
Test: atest VtsHalSensorsV2_0Target
Change-Id: Ib5f8fd513313670e88c2b973c1ff724658914eb2