Prevent NPE via null task info object that can be reset at any time by
other thread.

 - changed getTaskInfo() return value to Optional<TaskInfo>
 - removed misleading hasTaskInfo() API that should never be used

Change-Id: I6847d64f4f303fd2b2714e352567f534d7b55155
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>