Fix empty UID for legacy files
BUG FIX
Description of Bug and Fix
Legacy siafiles don't have the UID persisted in the metadata. Loading those files would cause them to have an empty string UID which causes problems in the siafileset.
Testing Added
TestUniqueIDMissing
Changes to Work Flow (if applicable)
Loading a siafile will now set the UID if it's an empty string.