safe default for is_valid

https://git.slub-dresden.de/digital-preservation/validate_workflows/-/blob/feature_bash_completion/validate_workflow.sh#L430 says:

is_valid=0

The safer alternative is to set is_valid=1 and make the validator explicitely confirm by its Return Code that a file is indeed valid. If there's a bug in the script or an issue with the file, then we can be certain that we don't get a false positive result for a file that wasn't actually checked (properly).