sh: Don't do anything if Enter is pressed with no command

apps/sh.c

@@ -90,6 +90,7 @@ int main()
         if (child == 0)
         {
             char** argv = split_command_into_argv(command);
+            if (argv[0] == NULL) return 0;
             sh_execvp(argv);
             perror(argv[0]);
             return 1;