fix: Make fields added in PR ocaml-sf#320 optional

This ensures learn-ocaml-client.master is still able to query a server
version learn-ocaml.0.12.

Minimal working example:
  cd .../learn-ocaml
  docker run --name=locaml -d -it -p 8080:8080 -v "$PWD/demo-repository:/repository" ocamlsf/learn-ocaml:0.12
  TOKEN=$(docker logs locaml | grep -e 'Initial teacher token created:' | sed -e 's/^.*: //')
  # then
  git checkout 42d8127
  opam install . --deps-only --locked -y
  opam install -y opam-installer.$(opam config var opam-version)
  eval $(opam env)
  make
  make opaminstall
  learn-ocaml-client init -s http://localhost:8080 -t "$TOKEN"
  touch demo.ml
  learn-ocaml-client grade demo.ml

Output before the fix:
  Fatal error: exception Json_encoding.Cannot_destruct(_)
  Called from unknown location
  Called from unknown location
  Called from unknown location
  Called from unknown location
  Called from unknown location
  Called from unknown location
  Called from unknown location

Author: erikmd

src/repo/learnocaml_exercise.ml

@@ -37,8 +37,8 @@ let encoding =
        (req "test" string)
        (req "solution" string)
        (req "max-score" int)
-       (req  "depend" (option string))
-       (req  "dependencies" (list string)))
+       (opt "depend" (string))
+       (dft "dependencies" (list string) []))
 
 (* let meta_from_string m =
  *   Ezjsonm.from_string m