freebsd.mkDerivation: do not crash on packages not using FreeBSD src

This includes drm-kmod and drm-kmod firmware.

pkgs/os-specific/bsd/freebsd/pkgs/drm-kmod/package.nix

@@ -22,6 +22,9 @@ let
   fetchOptions = (lib.importJSON ./versions.json).${branch};
 in
 mkDerivation {
+  # this derivation is tricky; it is not an in-tree FreeBSD build but it is meant to be built
+  # at the same time as the in-tree FreeBSD code, so it expects the same environment. Therefore,
+  # it is appropriate to use the freebsd mkDerivation.
   pname = "drm-kmod";
   version = branch;
 

pkgs/os-specific/bsd/freebsd/pkgs/mkDerivation.nix

@@ -107,7 +107,9 @@ lib.makeOverridable (
     // {
       patches =
         (lib.optionals (attrs.autoPickPatches or true) (
-          freebsd-lib.filterPatches patches (attrs.extraPaths or [ ] ++ [ attrs.path ])
+          freebsd-lib.filterPatches patches (
+            attrs.extraPaths or [ ] ++ (lib.optional (attrs ? path) attrs.path)
+          )
         ))
         ++ attrs.patches or [ ];
     }