loosen is_null

src/rstd/nullable.rs

@@ -91,7 +91,9 @@ impl<'a, Ctx: Context<'a>, A: Mentionable<'a, Ctx>> Nullable<'a, Ctx, A> {
             Nullable::NotNull(point) => Some(point),
         }
     }
+}
 
+impl<'a, Ctx: Context<'a>, A: MentionableBase<'a, Ctx>> Nullable<'a, Ctx, A> {
     pub fn is_null(&self) -> bool {
         match self {
             Nullable::Null(_) => true,