Partial fix for bug #77903

Partial fix for bug #77903

In the hash position APIs, make sure we always advance to the next
non-undef element and not just when the position is 0 (similar to
what foreach does). This can happen when the position of an
ArrayIterator is one past its current end and a new element is
inserted not directly at that position because the array is packed.

There is still a bug here (as shown in the tests), but this is a
separate issue that also affects plain array iteration in foreach.