Fix bug for topic is changed to empty.

Fix bug for topic is changed to empty.

When the provider delete topic, consumer never callback changed when topic is empty.
In this patch, consumer will callback changed topic to upper layer
when topic is empty from not empty.

Change-Id: I41d0ad42f19298ce2773ab4c17d37cd7c1aa6b5e
Signed-off-by: KIM JungYong <jyong2.kim@samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/12167
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Chihyun Cho <ch79.cho@samsung.com>
Reviewed-by: Hun-je Yeon <hunje.yeon@samsung.com>
Reviewed-by: Uze Choi <uzchoi@samsung.com>