For a tie, since the reference goes the other way (the primary key of the knot is a foreign key in the tie), it cannot be null. Or in other words, you cannot have some rows in the tie that reference the knot and some that do not. However, the simple solution is to introduce a value representing the "Unknown" in the knot instead. Then the rows for which you do not currently know the value can reference that explicitly in the knot.
Of course, making the tie historized will allow you to change this fact over time, should the value become known.