When a package is "not available" for a given platform, the print-pkgnames will
(correctly) return an empty list. This makes tsort-set think that no package
is matching here.

This is bad because:
  * bulk builds results won't be triggered for this package (and appear as
    "unknown" instead of "cannot be built here" / "not available").
  * package sets without NULLGLOB option will incorrectly raise an error

Fix by using the default PKGNAME in tsort-set when print-pkgnames returns the
empty list.