libjsonverify: allow option for missing files.

When 'optional' is set to true:
- If file is missing, test pass (with logs)
- If file is present but empty, test fails
- If file is present but in wrong format, test fails

Test: vts_processgroup_validate_test
Bug: 123664216
Change-Id: Ib6640c3c1ce159c5cac068042af564d98c9bbb1f
1 file changed