shellcheck: fix usse -n instead of ! -z (SC2236 SC2237)

 - https://www.shellcheck.net/wiki/SC2236 -- Use -n instead of ! -z.
 - https://www.shellcheck.net/wiki/SC2237

 -- Use [ -n .. ] instead of ! [ -z ....

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>