additional checks for NULL in ucl_elt_append()

If elt was NULL, ucl_elt_append() could crash, instead return head as an implicit array
My bad, what it meant to do here:
If head is not an array, but a scalar, 'upgrade' it to an array of 1 element

Doesn't the same thing happen to elt if head = null? if elt is an array it would get truncated?