Fixed a problem where splatted nil would get converted to [nil].

Fixes #2926.

Previously, the JIT code was not checking for nil, when trying to cast
the stack value to an array, thus creating an array with just nil in it.
This fix ensures an empty array gets created instead.

With help from @dbussink, and recorded it live on an hangout!
Merge pull request #2938 from razielgn/splat-jit-fix

Fixed a problem where splatted nil would get converted to [nil].