Fix header-abi-diff skips diff when config.json is not found

Skip loading config.json when the config.json is not found so the
header-abi-diff will still perform abi diff.

Test: preform abi diff without config.json
Bug: 243328443
Change-Id: I92ebf7e7bfe6abb57bdefa211eaf604efaee517e
2 files changed