core: handle --log-target=null when calling systemd-shutdown

When shutting down, if systemd was started with --log-target=null,
systemd-shutdown was being called with --log-target=console.

(cherry picked from commit 10f00ff17b)

src/core/main.c

@@ -2001,6 +2001,10 @@ finish:
                         command_line[pos++] = "kmsg";
                         break;
 
+                case LOG_TARGET_NULL:
+                        command_line[pos++] = "null";
+                        break;
+
                 case LOG_TARGET_CONSOLE:
                 default:
                         command_line[pos++] = "console";