file: don't ignore all --image flag errors
file: don't ignore all --image flag errors

Prior to this change, invalidly formatted `--image` flags would not
error, while I believe the logic was meant to skip empty image flags
only.

dgonyeo/docker2aci,dgonyeo/docker2aci,OMWAVE/docker2aci,appc/docker2aci,OMWAVE/docker2aci,appc/docker2aci