what result to return if the cell content type is formula.
Cleans up the confusion which was introduced with commit
8a73799d.

Change-Id: Ie4346173abf6bc3732d2fec06341f4e32cf68b82
Reviewed-on: https://gerrit.libreoffice.org/48725Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Eike Rathke <erack@redhat.com>