Fix test bug in auto_date_histo

The test would try to prepare a `Rounding` even when there aren't any
buckets. This would fail because there is no range over which to prepare
the rounding. It turns out that we don't need the rounding in that case
so we just use `null` then.

Closes #59131

server/src/test/java/org/elasticsearch/search/aggregations/bucket/histogram/InternalAutoDateHistogramTests.java

@@ -155,7 +155,7 @@ protected void assertReduced(InternalAutoDateHistogram reduced, List<InternalAut
 
         int roundingIndex = reduced.getBucketInfo().roundingIdx;
         RoundingInfo roundingInfo = AutoDateHistogramAggregationBuilder.buildRoundings(null, null)[roundingIndex];
-        Rounding.Prepared prepared = roundingInfo.rounding.prepare(lowest, highest);
+        Rounding.Prepared prepared = totalBucketConut > 0 ? roundingInfo.rounding.prepare(lowest, highest) : null;
 
         long normalizedDuration = (highest - lowest) / roundingInfo.getRoughEstimateDurationMillis();
         int innerIntervalIndex = 0;