fdo#77056: Treat empty cells as if they have a value of 0.0.

Change-Id: Ibe64cf7177a5298c1878e0014c049dc9c82b1344
(cherry picked from commit 83a88b942134314e86ac612d0ef70a8e4919e4af)
Reviewed-on: https://gerrit.libreoffice.org/9298
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>