WW-2225 Check for null error and messages on templates

Files:
modified: /struts/struts2/trunk/core/src/test/java/org/apache/struts2/views/jsp/ui/ActionErrorTagTest.java (try)
modified: /struts/struts2/trunk/core/src/main/resources/template/simple/actionmessage.ftl (try)
modified: /struts/struts2/trunk/core/src/main/resources/template/simple/actionerror.ftl (try)
modified: /struts/struts2/trunk/core/src/test/java/org/apache/struts2/views/jsp/ui/ActionMessageTagTest.java (try)