pid1: process zero-length notification messages again

This undoes 531ac2b. I acked that patch without looking at the code
carefully enough. There are two problems:
- we want to process the fds anyway
- in principle empty notification messages are valid, and we should
  process them as usual, including logging using log_unit_debug().

src/core/manager.c

@@ -1657,13 +1657,12 @@ static int manager_dispatch_cgroups_agent_fd(sd_event_source *source, int fd, ui
        return 0;
}

static void manager_invoke_notify_message(Manager *m, Unit *u, pid_t pid, const char *buf, size_t n, FDSet *fds) {
static void manager_invoke_notify_message(Manager *m, Unit *u, pid_t pid, const char *buf, FDSet *fds) {
        _cleanup_strv_free_ char **tags = NULL;

        assert(m);
        assert(u);
        assert(buf);
        assert(n > 0);

        tags = strv_split(buf, "\n\r");
        if (!tags) {
@@ -1725,10 +1724,6 @@ static int manager_dispatch_notify_fd(sd_event_source *source, int fd, uint32_t
                 * example... */
                return 0;
        }
        if (n == 0) {
                log_debug("Got zero-length notification message. Ignoring.");
                return 0;
        }

        CMSG_FOREACH(cmsg, &msghdr) {
                if (cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SCM_RIGHTS) {
@@ -1765,25 +1760,27 @@ static int manager_dispatch_notify_fd(sd_event_source *source, int fd, uint32_t
                return 0;
        }

        /* The message should be a string. Here we make sure it's NUL-terminated,
         * but only the part until first NUL will be used anyway. */
        buf[n] = 0;

        /* Notify every unit that might be interested, but try
         * to avoid notifying the same one multiple times. */
        u1 = manager_get_unit_by_pid_cgroup(m, ucred->pid);
        if (u1) {
                manager_invoke_notify_message(m, u1, ucred->pid, buf, n, fds);
                manager_invoke_notify_message(m, u1, ucred->pid, buf, fds);
                found = true;
        }

        u2 = hashmap_get(m->watch_pids1, PID_TO_PTR(ucred->pid));
        if (u2 && u2 != u1) {
                manager_invoke_notify_message(m, u2, ucred->pid, buf, n, fds);
                manager_invoke_notify_message(m, u2, ucred->pid, buf, fds);
                found = true;
        }

        u3 = hashmap_get(m->watch_pids2, PID_TO_PTR(ucred->pid));
        if (u3 && u3 != u2 && u3 != u1) {
                manager_invoke_notify_message(m, u3, ucred->pid, buf, n, fds);
                manager_invoke_notify_message(m, u3, ucred->pid, buf, fds);
                found = true;
        }