Fixed bug with unbalanced parens in literal strings

I faced a very weird behaviour where if I typed "(" and pressed enter in the REPL, the string would not be echoed, and in the *Messages* buffer of Emacs I would get this error:
```
clojure-string-start: Wrong type argument: characterp, nil
```

This was true for both my emacs setups on 2 machines, but none of my 6 colleagues could reproduce it. I ran the elisp debugger and discovered that the character in `clojure-string-start` was nil. The fix involves the addition of a simple nil check.

Please merge :-)