resolved rhbz#1101224 do not attempt to obtain names for NULL tabs
resolved rhbz#1101224 do not attempt to obtain names for NULL tabs

This happened when the HTML export via clipboard tried to resolve
conditional formats, where we have a temporary instance of a document
containing only the sheet to be exported.

Change-Id: Ic7498a1cab3eabede74773868287a2cc3edef052

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