fix: use [ -n ... ] rather than ! [ -z ...]

Style fix.

Courtesy of https://www.shellcheck.net/

Author: rouilj

scripts/Docker/roundup_start

@@ -16,7 +16,7 @@ trap "echo 'Exiting on pipefail'" ERR
 
 set -e
 
-if ! [ -z "$SHELL_DEBUG" ]; then
+if [ -n "$SHELL_DEBUG" ]; then
     set -xv
 fi