Fix #EZP-20434 - LocalFileService::remove accepts empty paths, causing recursive delete of install root

This is a fix proposal for https://jira.ez.no/browse/EZP-20434

Admittedly it's a hammer-handed approach, perhaps a more ideal fix would be to make sure an empty path does not actually reach `LocalFileService::remove`, i.e, through the `ImageStore::deleteFieldData` method. Nonetheless, this PR is at least a pointer on where the problem lies.