NOJIRA: Fix null sender or subject

In some cases, if an email sender is null (wich can be the case if the email is a test email sent via telnet for example),  a NPE is thrown by method getSenderName() in org.jasig.portlet.emailpreview.EmailMessage class. Fix the problem by replacing a null value by "-" String for Sender. Maybe a localized message "no sender" or "no subject" will be a more appropriate fix. A null subject is displayed "null" in the application.