IMPALA-769: Add empty string checks to StringParser functions

Change-Id: Id8f1ace64f90b0f2f47faa5d9fa6b8c602eafbad
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1369
Reviewed-by: Skye Wanderman-Milne <skye@cloudera.com>
Tested-by: jenkins