res_pjsip_pubsub: Add body_type to test_handler for unit tests

The ast_sip_subscription_handler "test_handler" used for the unit
tests didn't set "body_type" so the NULL value was causing
a SEGV in build_subscription_tree().  It's now set to "".

Resolves: #335

res/res_pjsip_pubsub.c

@@ -5171,6 +5171,7 @@ struct ast_sip_notifier test_notifier = {
  */
 struct ast_sip_subscription_handler test_handler = {
 	.event_name = "test",
+	.body_type = "",
 	.notifier = &test_notifier,
 };