Make GraphQL parameters strict following Absinthe 1.7 validating

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

test/graphql/resolvers/post_test.exs

@@ -64,7 +64,7 @@ defmodule Mobilizon.GraphQL.Resolvers.PostTest do
   """
 
   @create_post """
-  mutation CreatePost($title: String!, $body: String, $attributedToId: ID!, $draft: Boolean) {
+  mutation CreatePost($title: String!, $body: String!, $attributedToId: ID!, $draft: Boolean) {
     createPost(title: $title, body: $body, attributedToId: $attributedToId, draft: $draft) {
       ...PostFragment
     }