handle_tool_tip() previously used event->tool->data without checking
for NULL. When launching sway for the first time after a reboot, and
then tapping with a USI 2.0 stylus, the tool tip event is fired before
the tool proximity event. As event->tool is initialized during the tool
proximity handler handle_tool_proximity(), this was causing a crash.
The fix adds a NULL check before accessing the fields of
event->tool->data. In case of a NULL, a log message is emitted
indicating that the tool tip event fired before proximity. This logic is
identical to the logic in handle_tool_axis().
Fixes issue #8907
f6e5c29621