Fix hydra eval error

Author: hamishmack

builder/make-config-files.nix

@@ -5,7 +5,7 @@
 let
   # Sort and remove duplicates from nonReinstallablePkgs.
   # That way changes to the order of nonReinstallablePkgs does not require rebuilds.
-  nonReinstallablePkgs' = __attrNames (lib.genAttrs (component.pre-existing ++ nonReinstallablePkgs) (x: x));
+  nonReinstallablePkgs' = __attrNames (lib.genAttrs (component.pre-existing or [] ++ nonReinstallablePkgs) (x: x));
 
   ghc = if enableDWARF then defaults.ghc.dwarf else defaults.ghc;