Dataset creation fails if there's no default legal tag on Seismic DDMS sub-project

To reproduce this issue, create a sub-project without a default legal tag and run the DAG on a SEG-Y dataset in that sub-project.
The creation of the VDS dataset will fail.

The dataset creation should reuse the legal tag set on the SEG-Y dataset as it can differ from the default legal tag set in the sub-project.