Ensure that we can evaluate the platform attribute of ocaml packages.

pkgs/development/tools/ocaml/camlp5/5.15.nix

@@ -39,7 +39,7 @@ stdenv.mkDerivation {
     homepage = "${webpage}";
     license = stdenv.lib.licenses.bsd3;
     branch = "5";
-    platforms = ocaml.meta.platforms;
+    platforms = ocaml.meta.platforms or [];
     maintainers = [
       stdenv.lib.maintainers.z77z
     ];

pkgs/development/tools/ocaml/camlp5/default.nix

@@ -33,7 +33,7 @@ stdenv.mkDerivation {
     '';
     homepage = http://pauillac.inria.fr/~ddr/camlp5/;
     license = licenses.bsd3;
-    platforms = ocaml.meta.platforms;
+    platforms = ocaml.meta.platforms or [];
     maintainers = with maintainers; [
       z77z vbgl
     ];