eval-release.nix: Tolerate null

This will allow tests.pkg-config.defaultPkgConfigPackages to run on
hydra without breaking the tarball job.

Regarding the use of null, I'll quote 473ac96 which does lib.hydraJob.

By allowing null, we allow code to avoid filterAttrs, improving
laziness in real world use cases.
Specifically, this strategy prevents infinite recursion errors,
performance issues and possibly other errors that are unrelated to
the user's code.

maintainers/scripts/eval-release.nix

···
         if (builtins.tryEval attrs.drvPath).success
         then { inherit (attrs) name drvPath; }
         else { failed = true; }
+      else if attrs == null then {}
       else { recurseForDerivations = true; } //
            mapAttrs (n: v: let path' = path ++ [n]; in trace path' (recurse path' v)) attrs
     else { };