Fixed: empty-body clauses in COND should evaluate to the result of the test.

http://www.lispworks.com/documentation/HyperSpec/Body/m_cond.htm says:

> cond {clause}\* => result\*
> clause ::= (test-form form\*) 
>
> [...]
> 
> Test-forms are evaluated one at a time in the order in which they are given
> in the argument list until a test-form is found that evaluates to true.
>
> **If there are no forms in that clause, the primary value of the test-form is**
> **returned by the cond form.**

So, e. g., if we (defun foo () #xF00), then (cond ((foo))) from the
empty-cond-clause test shall evaluate to #xF00.  The translated JavaScript
expression evaluates to null no matter what; and that's what I'm trying to fix
here.