sensors:2.0:vts fix functional test for typeAsString field

For sensors of known type defined in SensorType (value <
DEVICE_PRIVATE_BASE), typeAsString field can be an empty string since
framework overrides it anyways.
The empty string test should fails only if type is >=
DEVICE_PRIVATE_BASE.
The matching type with typeAsString test is instead executed only if
the string is not empty (if it's not an empty string let's check that
at least is set to the right value).

Bug: 155514483
Test: atest VtsHalSensorsV2_0TargetTest
Signed-off-by: Denis Ciocca <denis.ciocca@gmail.com>
Merged-In: Ib5f8fd513313670e88c2b973c1ff724658914eb2
Change-Id: I4267ff6a158a571ff43122bdb7a38c9fa38f5900