cudaPackages.cudnn: fix source fetching

Currently, cudaPackages_10_0.cudnn.src does not build, because passing
`hash = null;` to fetchurl causes it to try to fetch in the sandbox,
where it does not have internet access.

pkgs/development/libraries/science/math/cudnn/generic.nix

@@ -21,12 +21,12 @@
 
 { fullVersion
 , url
-, hash ? null
-, sha256 ? null
+, hash ? ""
+, sha256 ? ""
 , supportedCudaVersions ? [ ]
 }:
 
-assert (hash != null) || (sha256 != null);
+assert (hash != "") || (sha256 != "");
 
 assert useCudatoolkitRunfile || (libcublas != null);