LUCENE-7787: HeatmapFacetCounter Bits.MatchNoBits optimization

lucene/CHANGES.txt

@@ -102,6 +102,11 @@ Improvements
 * LUCENE-7782: OfflineSorter now passes the total number of items it
   will write to getWriter (Mike McCandless)
 
+Optimizations
+
+* LUCENE-7787: spatial-extras HeatmapFacetCounter will now short-circuit it's
+  work when Bits.MatchNoBits is passed. (David Smiley)
+
 Other
 
 * LUCENE-7754: Inner classes should be static whenever possible.

lucene/spatial-extras/src/java/org/apache/lucene/spatial/prefix/HeatmapFacetCounter.java

@@ -142,6 +142,9 @@ public class HeatmapFacetCounter {
     }
 
     final Heatmap heatmap = new Heatmap(columns, rows, ctx.makeRectangle(heatMinX, heatMaxX, heatMinY, heatMaxY));
+    if (topAcceptDocs instanceof Bits.MatchNoBits) {
+      return heatmap; // short-circuit
+    }
 
     //All ancestor cell counts (of facetLevel) will be captured during facet visiting and applied later. If the data is
     // just points then there won't be any ancestors.