Do not write initial empty values for the diagnostic properties.

There is a separate step that initializes them with meaningful values.

Change-Id: I2480a029773b62b0e77b8c52c73f2ee465caf17c
Fixes: b/65125128
Test: runtest -x packages/services/Car/tests/android_car_api_test/src/android/car/apitest/CarDiagnosticManagerTest.java