LPS-33695 - Update taglib cssClass to StringPool.Blank instead of null

Hey Shinn,

Can you review these changes for updating the taglib cssClass to default to StringPool.Blank instead of a null value.  This update can be found at http://issues.liferay.com/browse/LPS-33695.

Please let me know if you have any questions.

Thanks!

- Jon Mak