Hi!

I had a look and apparently cider-eval returns nil when the expression
is a comment, vs the string "nil" when the Clojure expression returns nil.
Simply filtering nil values out seems to have fixed the issue
for me.
@Christopher, can you check the attached patch if this works for you?

@Ihor, a few developer related questions:
- I wanted to add a test, how can I run only test-ob-clojure?
  I don't think the tests work currently and I would fix them and add more.
- What should I use for indention? Looking in ob-clojure and org.el
  it seems that using tabs with tab-width 8 is standard but there's
  quite a few lines where it's spaces only.
- If this simple patch for example or fixing tests is good,
  should I just install it?
  Can I "freely" install minor changes to ob-clojure or should
  everything go over the mailinglist?

Thanks. I hope I can work a bit more on ob-clojure the next few days,
I think I missed a few emails, sorry. Now I have a filter rule where
mails with Clojure in the subject go to my inbox.

Cheers,
  Daniel