[Bug libstdc++/90299] std::filesystem::absolute("") and std::filesystem::absolute("", ec) behave differently

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90299

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
And for the avoidance of doubt, the problem is not that !exists(p) (although
that is true) but that an empty path doesn't refer to any file system location.
absolute("does not exist") is not an error, but absolute("") is.

There is no absolute representation of the empty path, it's a category error.