More idiomatic change of empty mono type

Author: rozza

driver-reactive-streams/src/main/com/mongodb/reactivestreams/client/internal/gridfs/GridFSUploadPublisherImpl.java

@@ -27,6 +27,7 @@
 import com.mongodb.reactivestreams.client.ClientSession;
 import com.mongodb.reactivestreams.client.MongoCollection;
 import com.mongodb.reactivestreams.client.gridfs.GridFSUploadPublisher;
+import org.bson.BsonDocument;
 import org.bson.BsonValue;
 import org.bson.Document;
 import org.bson.types.Binary;
@@ -155,7 +156,7 @@ private Mono<Void> createCheckAndCreateIndexesMono(@Nullable final Timeout timeo
                 .switchIfEmpty(Mono.defer(() ->
                         checkAndCreateIndex(filesCollection.withReadPreference(primary()), FILES_INDEX, timeout)
                                 .then(checkAndCreateIndex(chunksCollection.withReadPreference(primary()), CHUNKS_INDEX, timeout))
-                                .then(Mono.fromCallable(Document::new))
+                                .then(Mono.empty())
                 ))
                 .then();
     }