Merge pull request #172786 from danielfullmer/cudnn-src

cudaPackages.cudnn: fix source fetching

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

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