avoid unnecessary object retention in FacetRangeProcessor

• The (private) doSubs method is a no-op if there are no sub-facets.
• The (private) intersections and filters arrays are only used by the doSubs method.
• The (private) rangeStats method currently always populates the intersections and filters arrays, even when nothing actually subsequently uses them.
• If rangeStats only populated the intersections array when it's actually needed then the DocSet intersection object would remain local in scope and hence the garbage collector could collect it earlier.

https://github.com/apache/lucene-solr/blob/releases/lucene-solr/8.7.0/solr/core/src/java/org/apache/solr/search/facet/FacetRangeProcessor.java#L531-L555