Allow empty S3 storage class when listing

Minio returns an empty element.

Author: gaul

apis/s3/src/main/java/org/jclouds/s3/xml/ListMultipartUploadsHandler.java

@@ -96,7 +96,8 @@ public void endElement(String uri, String name, String qName) {
       } else if (qName.equals("UploadId")) {
          uploadId = currentOrNull(currentText);
       } else if (qName.equals("StorageClass")) {
-         storageClass = ObjectMetadata.StorageClass.valueOf(currentOrNull(currentText));
+         String currentValue = currentOrNull(currentText);
+         storageClass = currentValue != null ? ObjectMetadata.StorageClass.valueOf(currentValue) : ObjectMetadata.StorageClass.STANDARD;
       } else if (qName.equals("Initiated")) {
          initiated = dateParser.iso8601DateOrSecondsDateParse(currentOrNull(currentText));
       } else if (qName.equals("Upload")) {