sage: If docs are not enabled then don't include a doc attribute

there should be no doc attribute if docs are not enabled

the doc attribute is used in nixos/documentation.nix

https://github.com/NixOS/nixpkgs/blob/adeb7629fc6ccf986ccc7013efc60d5e6d5269f1/nixos/modules/misc/documentation.nix#L342

the doc will still be built because the doc attribute exists

pkgs/applications/science/math/sage/sage.nix

···
   passthru = {
     tests = sage-tests;
     quicktest = sage-tests.override { longTests = false; timeLimit = 600; }; # as many tests as possible in ~10m
-    doc = sagedoc;
     lib = sage-with-env.env.lib;
     with-env = sage-with-env;
     kernelspec = jupyter-kernel-definition;
+  } // lib.optionalAttrs withDoc {
+    doc = sagedoc;
   };
 
   meta = with lib; {