nspawn: restore journal directory is empty check

This undoes part of commit e6a4a517be.

Instead of removing the error message about non-empty journal bind mount
directories, simply downgrade the message to a warning and proceed.

src/nspawn/nspawn.c

@@ -1143,6 +1143,9 @@ static int setup_journal(const char *directory) {
         } else if (access(p, F_OK) < 0)
                 return 0;
 
+        if (dir_is_empty(q) == 0)
+                log_warning("%s is not empty, proceeding anyway.", q);
+
         r = mkdir_p(q, 0755);
         if (r < 0) {
                 log_error("Failed to create %s: %m", q);