fix passing null routing to Elasticsearch's builder.

Closes #3278

Signed-off-by: Peter-Josef Meisch <pj.meisch@sothawo.com>

Author: sothawo

src/main/java/org/springframework/data/elasticsearch/client/elc/RequestConverter.java

@@ -589,10 +589,13 @@ public IndexRequest<?> documentIndexRequest(IndexQuery query, IndexCoordinates i
 			builder.version(query.getVersion()).versionType(versionType);
 		}
 
-		builder //
-				.ifSeqNo(query.getSeqNo()) //
-				.ifPrimaryTerm(query.getPrimaryTerm()) //
-				.routing(query.getRouting()); //
+		builder
+				.ifSeqNo(query.getSeqNo())
+				.ifPrimaryTerm(query.getPrimaryTerm());
+
+		if (query.getRouting() != null) {
+			builder.routing(query.getRouting()); //
+		}
 
 		if (query.getOpType() != null) {
 			switch (query.getOpType()) {