Add POP_ARRAY and POP_ARRAY_NE.

These pop from the assembly stack, and check that what they are popping
is an NSArray.  If it's NSNull instead, then the result is nil.

POP_ARRAY_NE checks whether the array is non-empty, and if not it returns
nil too.
Add PUSH_NULL, which is a simple alias for PUSH([NSNull null]).