Text editors: Do not show tool tip for empty area

The editors were showing tool tips for the last cursor position in a
line, even if the mouse was hovered over the empty space behind the
line.

Task-number: QTCREATORBUG-9437
Change-Id: Ibe3b0140b73c3a15811eeaf50a15d75f378f37f4
Reviewed-by: David Schulz <david.schulz@digia.com>