When estimating an $elemMatch predicate, the requirements map of a Sargable node may include a special 'PathArr' predicate that signals we should estimate this field using $elemMatch semantics. This predicate may be split out into its own SargableNode during the MemoExploration phase.

When this happens, we need a special case in order to get a correct estimate when we see a Sargable node with just a requirements map entry like PathGet "a" PathIdentity [array interval]. We should use a counter of empty arrays in statistics to obtain a reasonable estimate for the proportion of arrays we expect in the output of the SargableNode.