cue: return empty struct for disallowed unification

The old code seems wrong.

Change-Id: I3d73c424613af198fab55458700c61c2d723ea52
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6582
Reviewed-by: CUE cueckoo <[email protected]>
Reviewed-by: Marcel van Lohuizen <[email protected]>

Author: mpvl

cue/types.go

@@ -1628,7 +1628,7 @@ func (v Value) Template() func(label string) Value {
 		if val, _ := x.optionals.constraint(ctx, arg); val != nil {
 			return remakeValue(v, val)
 		}
-		return v
+		return Value{}
 	}
 }