[1.6.x] Fixed #20660 -- Do not try to delete an unset FieldFile

Thanks stanislas.guerra at gmail.com for the report and
Baptiste Mispelon for the review.
Backport of ea3fe78a9d from master.
[1.6.x] Do not allow FileSystemStorage.delete to receive an empty name

Refs #20660.
Backport of 7fbab3eba from master.