Remove cleared placeholders when deleting list suffixes

[muneikh]

Deleting all list elements currently leaves trailing None placeholders: dpath.delete({'items': [0, 1, 2]}, 'items/*') produces {'items': [None, None]}. The same happens when deleting a contiguous suffix or entries in nested lists.

Track the positions cleared by this delete call. When removing a list tail, remove any newly cleared positions that have become trailing positions too. Original None values and indices of retained entries remain intact, and traversal order stays unchanged.

Regression tests cover entire lists, suffixes, nested lists, interior holes, and pre-existing None values. Three original regressions fail before the change. Full nose2/Hypothesis suites pass through tox on Python 3.10 and 3.13.