bake_flatcar_image: Fix check for empty sysext list

"${sysexts[@]}" expands every argument separately, while we need all
array members to be expanded into a single string for the test to work.
"${sysexts[*]}" is what we want.

Signed-off-by: Jeremi Piotrowski <[email protected]>

bake_flatcar_image.sh

@@ -273,7 +273,7 @@ while [ $# -gt 0 ]; do
     esac
 done
 
-if [ -z "${sysexts[@]}" ] ; then
+if [ -z "${sysexts[*]}" ] ; then
     echo -e "\nERROR: No sysexts specified.\n"
     print_help
     exit 1