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

OOXML validation spewed
- (xl/workbook.xml:2 col:700) cvc-complex-type.2.4.b: The content of
  element ´externalReferences´ is not complete. One of
  ´{"http://schemas.openxmlformats.org/spreadsheetml/2006/main":externalReference}´
  is expected.

Change-Id: I80552a0e9ffcc201d0462ac20ce37f6657e43587