* Added breaking change section for GROUP BY behavior: now it conside…

…rs null or empty values as a separate group/bucket. Previously, they were ignored.

* This is part of backporting of #32832

docs/reference/migration/migrate_6_5.asciidoc

@@ -5,6 +5,7 @@ This section discusses the changes that you need to be aware of when migrating
 your application to Elasticsearch 6.5.
 
 * <<breaking_65_logging_changes>>
+* <<breaking_65_sql_changes>>
 
 See also <<release-highlights>> and <<es-release-notes>>.
 
@@ -23,3 +24,13 @@ Elasticsearch will log a warning on startup and log with the new pattern. It
 will not change the logging configuration files though. You should make this
 change before 7.0 because in 7.0 Elasticsearch will no longer automatically
 add the node name to the logging configuration if it isn't already present.
+
+[[breaking_65_sql_changes]]
+=== SQL plugin changes
+
+==== Grouping by columns with missing values will create an additional group 
+
+An additional group will be present in the result of requests containing a
+`GROUP BY` for a column that has missing values in the returned documents.
+The records with missing values in the grouped by column will be collectively
+considered a single bucket.