Fixed: constructing a 'RowCountChanged' notification generates a nil warning when dragging a row

Previously, when

- Dragging a row in the rule editor

would eventually try to generate a dictionary with key 'indexes' and a
nil value in the method __postRowCountChangedNotificationOfType:indexes:.
Generating an empty dictionary instead when there is no indexes solves
this issue.
Merge pull request #1881 from kerusan/cpruleeditor_post_row_change

Fixed: 'RowCountChanged' notification generates nil warning when dragging a row