ensure valid iterator even with empty content.
ensure valid iterator even with empty content.

Change-Id: Icd2e193b66ace8212d1af086192abf0c43c5953f

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core