There’s something about having an issue, googling to find a result, and discovering you had the same issue 7 years ago, and probably have the same answer today. Extending the timer a little, since obviously my rules files have grown. And AI on the internet also suggested directly importing:
import org.openhab.core.types.*
which might help it to recognise NULL earlier. It complains that wildcard imports are deprecated, but the combination of the two worked.