L3_Templates: tr_GMM_DET_REQ_MT: use * instead of omit for `cause`

The omit force this field to be not present, while a * allows to be present or not.
As user of this tr I would expect to ignore this field rather than an explicit omit.

Change-Id: Iae91f752789273934a6382bdd474594c3c50bbe9
1 file changed