fdo#74747: Correctly inspect formula result value for xlsx export.

Change-Id: I757a8eb371b432970885e2fbd6aea9dd965ab5c0
(cherry picked from commit c1dc7576c18cc534e1934459f5fb210091a5b484)
Reviewed-on: https://gerrit.libreoffice.org/8527
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>