Fix warning for rkp_factory_extraction_tool

The empty string was handled as a value. Also, the unit test that tested
this functionality was fixed.

Bug: 377389989
Test: atest system/security/provisioner
Test: m rkp_factory_extraction_tool
Test: adb push rkp_factory_extraction_tool /data
Test: adb shell adb shell /data/rkp_factory_extraction_tool
Test: The above command does not produce a warning.
Change-Id: Ifd12bb43956be00efce0b45656da6d65e19c02fa
3 files changed