From the Native histograms spec:

histogram_quantile treats observations of value NaN (which SHOULD NOT happen, see above) effectively as observations of +Inf. This follows the rationale that NaN is never less than any value that histogram_quantile returns and is consistent with how classic histograms usually treat NaN observations (which end up in the +Inf bucket in most implementations). (TODO: The correct implementation of this behavior still needs to be verified by tests.)