lib.hydraJob: Tolerate null

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.

lib/customisation.nix

@@ -252,7 +252,8 @@ rec {
       outputsList = map makeOutput outputs;
 
       drv' = (lib.head outputsList).value;
-    in lib.deepSeq drv' drv';
+    in if drv == null then null else
+      lib.deepSeq drv' drv';
 
   /* Make a set of packages with a common scope. All packages called
      with the provided `callPackage` will be evaluated with the same