Relax the constraint that a `NestedLocation` must have a nested entry
name specified so that URLs can be split and rebuilt.
Prior to this commit, given a URL of the following form:
jar:nested:/myjar.jar!/nested.jar!/my/file
It was possible to create a FileSystem from
"jar:nested:/myjar.jar!/nested.jar" and from that create a path to
"my/file".
However, it wasn't possible to create a FileSystem from
"jar:nested:/myjar.jar", then create another file system from the path
"nested.jar" and then finally create a path to "/nested.jar".
This was because `nested:/myjar.jar` was not considered a value URL
because it didn't include a nested entry name.
Projects such as `JobRunr` were relying on the ability to compose file
systems, so it makes sense to remove our somewhat artificial
restriction.
Fixes gh-38592
6fd691af58