fdo#77735: Don't proceed when all cells in the range are empty.

That means there is nothing to delete, and proceeding would cause Calc
to freeze.

Change-Id: I2a8fb5736870ba459082873c8f864283d8b9c664
(cherry picked from commit bc7e0967be52f5eb7948fbb1c30edc7dd5acc18d)
Reviewed-on: https://gerrit.libreoffice.org/9388
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>