Only set transcoder if tc != NULL

Crashes otherwise (for example when creating admin instances)

Change-Id: Ieb32d27eaf272ddb5517e53fab81d5423ea87248
Reviewed-on: http://review.couchbase.org/27093
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>