fdo#35142: Avoid importing unnamed styles from XLS.
fdo#35142: Avoid importing unnamed styles from XLS.

This actually causes undefined behavior since every cell is supposed
to be associated with some style.  One thing affected by this is ODS
export (as described in the bug).

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