In this case, I would do it as a 2-phase lookup. First with the stricter condition, and for remaining empty values - with the looser one.
See the example below.
2-phase-lookup.morph (4.5 KB)
Book1.xlsx (9.9 KB)
It can be possible to generalize the approach: do all the lookups corresponding to parts of the lookup condition, then use the “Rule” action as in the example to pick the correct lookup value.