Updated filter checks since it cannot be null.

Introducing a default of "" for the filter QueryParam in BackupServlet.list had the side effect that any calls to /backup/list without specifying a filter would fail with "java.lang.IllegalArgumentException: No enum constant com.netflix.priam.backup.AbstractBackupPath.BackupFileType".
Merge pull request #328 from decbis/master

Updated filter checks since it cannot be null.