Fix some issues of GroupSynchronization

Fix some issues of GroupSynchronization
	- findGroup() with null callback parameter issue is fixed.
	  : return OC_STACK_ERROR, when callback parameter is passed with a null value.
	- joinGroup() failure when there is no group match with group type parameter is fixed.
	- deleteGroup() failure when delete group with no child is fixed.

Erase "else" statement for guard condition in findCallback NULL checking.

Change-Id: Id00160e3b2e85e03c4e4dd5bd5fb401f6a8bbd42
Signed-off-by: Soyoung Youn <sy01.youn@samsung.com>