pypy3Packages.zarr.optional-dependencies: fix the eval

Without the chnage the eval fails as:

    $ nix-instantiate -A pypy3Packages.zarr.optional-dependencies
    error:
       … while evaluating the attribute 'docs'
         at pkgs/development/python-modules/zarr/default.nix:126:7:
          125|       ];
          126|       docs = [
             |       ^
          127|         # Doc building

       … while selecting an attribute
         at pkgs/development/python-modules/zarr/default.nix:145:10:
          144|       ++ mkdocs-material.optional-dependencies.imaging
          145|       ++ markdown-exec.optional-dependencies.ansi
             |          ^
          146|       ++ numcodecs.optional-dependencies.msgpack;

       error: expected a set but found null: null

pkgs/development/python-modules/zarr/default.nix

@@ -142,7 +142,7 @@ buildPythonPackage (finalAttrs: {
         #pytest
       ]
       ++ mkdocs-material.optional-dependencies.imaging
-      ++ markdown-exec.optional-dependencies.ansi
+      ++ lib.optionals (markdown-exec != null) markdown-exec.optional-dependencies.ansi
       ++ numcodecs.optional-dependencies.msgpack;
     };
   };