[mk] Let tsort-set script still return a package name when none is available
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.