MultiValueMode does two things at once: it selects the min/max/median/sum value when documents have multiple values, but also returns a missing value when documents do not have a value. Decoupling would make things cleaner, easier to test and would eg. allow to remove the hack from the matrix stats aggs that need to treat NEGATIVE_INFINITY as a missing value since we do not expose ways to select the minimum value without also replacing missing values at the moment.