Initialize the union selector of undefined upsilon nodes
While we're guaranteed never to look at undefined values,
we're not guaranteed that the type information can't change, which
in the case of a union generates code to change the layout.
As a result, we need to make sure to have the union selectors
be valid (the actual data will be junk and no semantic operation
will ever look at it, but it shouldn't be ouf of bounds).
Ref #29152
