Return undef instead of null on toConstElem failure.

This fixes a segfault with associative array literals of
arrays of associative array literals, which occured because
of arrayLiteralToConst not handling null values properly.
Ensuring that null pointers are handled correctly in all
toConstElem callers is much more error-prone than just
returning an LLVM undef, an error is emitted anyway.

The root of the problem is actually in the kludgly
implementation of AssocArrayLiteral::toElem, we should
revisit this at some point.