An undervote is not necessarily a blemished or invalidated vote. “NOTA” is not included as an alternative to the choice offered, which may not be acceptable at all to the specific voter, so that selection was deliberately not made.
Lots of people don’t vote for a certain candidate.
I refused vote to for the only Democrat for county treasurer listed on the ballot.
Undervotes are perfectly legitimate. You don’t have to vote for a candidate at all.