Retouche docker-compile-and-run

test~coding~readingCppPrograms.fr/docker-compile-and-run

@@ -12,7 +12,7 @@ usage () {
     echo The exit status is that of the compiler.
 }
 
-if [ -n $1 ]; then
+if [ x"$1" = "x" ]; then
     usage
     exit 0
 fi