really do not write empty externalReferences element, fdo#45286 follow-up
really do not write empty externalReferences element, fdo#45286 follow-up

Xti records exist also for internal sheet references in BIFF. It is not
enough to check for their presence, at least one EXC_SBTYPE_EXTERN must
be present as SUPBOOK.

Change-Id: I5553a28412465d83fd965b09908261141c51ea35

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