Fix empty list as default value in OpInstanceMultiAlloc

Commit 12e62af5 (“Adding the new opcode for multi-allocation”)
introduced a "bad" default value; while porting this to Haskell, I
realised this is wrong.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>