rowCount() must return 0 when the parent is valid: this indicates that
the list's items has no children and thus it is no tree model. The
list's size has to be returned for an invalid parent (== the invisible
root node).

Change-Id: Ieaa884958ea0094aca2232160b6f769bf90514c4
Reviewed-by: Orgad Shaneh <orgads@gmail.com>