astyle: switch condition to empty string check

Thanks to @bkueang 's review comment I switched to an explicit check for
an empty sting instead of a condition that could be theoretically true
in other cases and is less readable.

Type "man test" on your terminal to read up what -n stands for.

Tools/astyle/check_code_style.sh

@@ -5,7 +5,7 @@ DIR=$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )
 
 if [ -f "$FILE" ]; then
 	CHECK_FAILED=$(${DIR}/fix_code_style.sh --dry-run --formatted $FILE)
-	if [[ $CHECK_FAILED ]]; then
+	if [ -n "$CHECK_FAILED" ]; then
 		${DIR}/fix_code_style.sh --quiet < $FILE > $FILE.pretty
 
 		echo