here is an AQL example that should solve your problem:
for f in Folders
filter LENGTH( EDGES(FolderRelations, v._id, "outbound")) == 0
return f
you will get a list of all vertices that have no folder above in the hierarchy.
but be aware: saving {key:1} will not have the desired effect, you have to set:
{_key: "1"}
_key is used for internal key attribute, and it has to be a string.