elm layout: Fixed handling of NULL in part_content_set.
elm layout: Fixed handling of NULL in part_content_set.

Signed-off-by: Daniel Zaoui <daniel.zaoui@samsung.com>

SVN revision: 69918

rvandegrift/elementary,FlorentRevest/Elementary,rvandegrift/elementary,FlorentRevest/Elementary,tasn/elementary,rvandegrift/elementary,rvandegrift/elementary,tasn/elementary,tasn/elementary,tasn/elementary,FlorentRevest/Elementary,FlorentRevest/Elementary,tasn/elementary