oox: Code protection against empty effects
oox: Code protection against empty effects

This is an extra check against clumsy programmers like me to prevent
generating incorrect documents.

Change-Id: I22261e3b6123a9a44461683519e33224f08adb5a

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