Validation rule of BKM typeref (bkm's variable element, typeref attribute)

In the following trivial example model:

The Validation warning indicates the model is missing the setup of a typeRef for the BKM.

But the typeref on the BKM is in conflict with what was discussed here: https://github.com/dmn-tck/tck/issues/112#issuecomment-362590717

For what concerning current DMN v1.1 validation, a fix will be provided to exclude BKMs typeref from the above Validation warning

i.e.: avoid Validation to raise the warning in the screenshot.