Prevent sync client from being created in reactive test config.

Closes #3817
Original pull request: #3987.

Author: christophstrobl

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/test/util/MongoTestTemplateConfiguration.java

@@ -65,7 +65,11 @@ MongoConverter mongoConverter() {
 
 		if (converter == null) {
 
-			converter = new MappingMongoConverter(new DefaultDbRefResolver(databaseFactory()), mappingContext());
+			if(dbFactoryConfig.syncClient != null || syncClient != null) {
+				converter = new MappingMongoConverter(new DefaultDbRefResolver(databaseFactory()), mappingContext());
+			} else {
+				converter = new MappingMongoConverter(NoOpDbRefResolver.INSTANCE, mappingContext());
+			}
 
 			if (mongoConverterConfigurer.customConversions != null) {
 				converter.setCustomConversions(mongoConverterConfigurer.customConversions);