LPS-38539 If task comments are null instert a blank string instead

its necessary to pass in a blank because later on in the execution we check for the value of WorkflowConstants.CONTEXT_TASK_COMMENTS regardless of whether anything was entered or not, so if you dont pass in anything you get a npe as explained here - http://issues.liferay.com/browse/LPS-38539?focusedCommentId=342657&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-342657

and again this only fixes the issue in 6.1.x. to fix it in master we will need to do something regarding the situation in this comment - http://issues.liferay.com/browse/LPS-38539?focusedCommentId=341837&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-341837