Fix date_histogram serialization bug

I caused a serialization bug in elastic#50873. It is kind of tricky to get:
* Run a date_histogram aggregation
* On a shard on 7.6
* That shard doesn't return any results
* The coordinating node is before 7.6

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/10_histogram.yml

@@ -196,3 +196,94 @@ setup:
   - match: { aggregations.histo.buckets.2.key_as_string: "2016-01-01T00:00:00.000Z" }
 
   - match: { aggregations.histo.buckets.2.doc_count: 2 }
+
+
+---
+"date_histogram":
+  - skip:
+      version: " - 7.1.99"
+      reason:  calendar_interval introduced in 7.2.0
+
+  - do:
+      indices.create:
+          index: test_2
+          body:
+            settings:
+              # There was a BWC issue that only showed up on empty shards. This
+              # test has 4 docs and 5 shards makes sure we get one empty.
+              number_of_shards: 5
+            mappings:
+              properties:
+                date:
+                  type : date
+
+  - do:
+      bulk:
+        index: test_1
+        refresh: true
+        body:
+            - '{"index": {}}'
+            - '{"date": "2016-01-01"}'
+            - '{"index": {}}'
+            - '{"date": "2016-01-02"}'
+            - '{"index": {}}'
+            - '{"date": "2016-02-01"}'
+            - '{"index": {}}'
+            - '{"date": "2016-03-01"}'
+
+  - do:
+      search:
+        body:
+          aggs:
+            histo:
+              date_histogram:
+                field: date
+                calendar_interval: month
+
+  - match: { hits.total.value: 4 }
+  - length: { aggregations.histo.buckets: 3 }
+  - match: { aggregations.histo.buckets.0.key_as_string: "2016-01-01T00:00:00.000Z" }
+  - match: { aggregations.histo.buckets.0.doc_count: 2 }
+  - match: { aggregations.histo.buckets.1.key_as_string: "2016-02-01T00:00:00.000Z" }
+  - match: { aggregations.histo.buckets.1.doc_count: 1 }
+  - match: { aggregations.histo.buckets.2.key_as_string: "2016-03-01T00:00:00.000Z" }
+  - match: { aggregations.histo.buckets.2.doc_count: 1 }
+
+---
+"date_histogram with offset":
+  - skip:
+      version: " - 7.1.99"
+      reason:  calendar_interval introduced in 7.2.0
+
+  - do:
+      bulk:
+        index: test_1
+        refresh: true
+        body:
+            - '{"index": {}}'
+            - '{"date": "2016-01-01"}'
+            - '{"index": {}}'
+            - '{"date": "2016-01-02"}'
+            - '{"index": {}}'
+            - '{"date": "2016-02-01"}'
+            - '{"index": {}}'
+            - '{"date": "2016-03-01"}'
+
+  - do:
+      search:
+        body:
+          aggs:
+            histo:
+              date_histogram:
+                field: date
+                calendar_interval: month
+                offset: +1d
+
+  - match: { hits.total.value: 4 }
+  - length: { aggregations.histo.buckets: 3 }
+  - match: { aggregations.histo.buckets.0.key_as_string: "2015-12-02T00:00:00.000Z" }
+  - match: { aggregations.histo.buckets.0.doc_count: 1 }
+  - match: { aggregations.histo.buckets.1.key_as_string: "2016-01-02T00:00:00.000Z" }
+  - match: { aggregations.histo.buckets.1.doc_count: 2 }
+  - match: { aggregations.histo.buckets.2.key_as_string: "2016-02-02T00:00:00.000Z" }
+  - match: { aggregations.histo.buckets.2.doc_count: 1 }

server/src/main/java/org/elasticsearch/search/aggregations/bucket/histogram/DateHistogramAggregator.java

@@ -146,7 +146,7 @@ public InternalAggregation buildAggregation(long owningBucketOrdinal) throws IOE
         // value source will be null for unmapped fields
         // Important: use `rounding` here, not `shardRounding`
         InternalDateHistogram.EmptyBucketInfo emptyBucketInfo = minDocCount == 0
-                ? new InternalDateHistogram.EmptyBucketInfo(rounding, buildEmptySubAggregations(), extendedBounds)
+                ? new InternalDateHistogram.EmptyBucketInfo(rounding.withoutOffset(), buildEmptySubAggregations(), extendedBounds)
                 : null;
         return new InternalDateHistogram(name, buckets, order, minDocCount, rounding.offset(), emptyBucketInfo, formatter, keyed,
                 pipelineAggregators(), metaData());