Prevent TTVA decos from running if a merging stream name is specified

Closes DATREP-206 (rather, the merging issues they see).

Tagging @goblirsc @vcairo @elmsheus - I don't have a working setup for testing if this actually resolves the issues. I think it should, but I also don't know if the merging stream name would be specified in non-merging contexts (in which case this fix would wrongly prevent the decorations from running)