Fix errors on empty stacktraces

[yuhan0]

Fixes #3022, succeeds #3023.

I also edited the docs to reflect that Cider no longer throws an error, and added beginner-friendly instructions for setting the JVM opts.
Note: only added examples for Clojure CLI, since Leiningen enables the flag by default and I'm not familiar with Boot.

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Thanks!

If you're just starting out to hack on CIDER you might find this section of its
manual extremely useful.

[yuhan0]

(I removed this clause since it's not clear that the stacktraces are only omitted for NPEs or cause them to occur)

[bbatsov]

Thanks!