Aggregations: if maxOrd is 0 then use noop collector

Before the OrdinalsCollector was used and this leads to a ArrayIndexOutOfBoundsException

Closes #6413

src/main/java/org/elasticsearch/search/aggregations/metrics/cardinality/CardinalityAggregator.java

@@ -93,6 +93,10 @@ public class CardinalityAggregator extends NumericMetricsAggregator.SingleValue
         if (bytesValues instanceof BytesValues.WithOrdinals) {
             BytesValues.WithOrdinals values = (BytesValues.WithOrdinals) bytesValues;
             final long maxOrd = values.ordinals().getMaxOrd();
+            if (maxOrd == 0) {
+                return new EmptyCollector();
+            }
+
             final long ordinalsMemoryUsage = OrdinalsCollector.memoryOverhead(maxOrd);
             final long countsMemoryUsage = HyperLogLogPlusPlus.memoryUsage(precision);
             // only use ordinals if they don't increase memory usage by more than 25%
@@ -166,6 +170,24 @@ public class CardinalityAggregator extends NumericMetricsAggregator.SingleValue
 
     }
 
+    private static class EmptyCollector implements Collector {
+
+        @Override
+        public void collect(int doc, long bucketOrd) {
+            // no-op
+        }
+
+        @Override
+        public void postCollect() {
+            // no-op
+        }
+
+        @Override
+        public void close() throws ElasticsearchException {
+            // no-op
+        }
+    }
+
     private static class DirectCollector implements Collector {
 
         private final LongValues hashes;

src/test/java/org/elasticsearch/search/aggregations/metrics/CardinalityTests.java

@@ -123,6 +123,13 @@ public class CardinalityTests extends ElasticsearchIntegrationTest {
         }
         indexRandom(true, builders);
         createIndex("idx_unmapped");
+
+        IndexRequestBuilder[] dummyDocsBuilder = new IndexRequestBuilder[10];
+        for (int i = 0; i < dummyDocsBuilder.length; i++) {
+            dummyDocsBuilder[i] = client().prepareIndex("idx", "type").setSource("a_field", "1");
+        }
+        indexRandom(true, dummyDocsBuilder);
+
         ensureSearchable();
     }