Update taglib cssClass to StringPool.Blank instead of null

• Type: Bug
• Status: Closed
• Resolution: Won't Fix
• Affects Version/s: 6.1.1 CE GA2, 6.1.20 EE GA2
• Fix Version/s: 6.1.1 CE GA2, 6.1.20 EE GA2
• Component/s: Dev Tools, Frontend Infrastructure > UI Taglibs
• Labels: None
• Branch Version/s:
  6.1.x
• Git Pull Request: https://github.com/brianchandotcom/liferay-portal/pull/10063

Update taglibs to set cssClass to StringPool.Blank instead of null since null is returned as the class to the page.jsp