[REVIEW 3-6 3-6-3] fix for fdo#54552, only use non blank cells in the visible data methods in ScColumn

[Markus Mohrhard]

Hey,

[1] fixes the problem that blank cells, like broadcaster, cells are
considerend non empty for the "visible" methods.

Regards,
Markus

[1] http://cgit.freedesktop.org/libreoffice/core/commit/?id=40377a6e26aa61a1c0788cad1c97a10911d38da8