systemd/system/ensure-sysext: skip unit if systemd-sysext is skipped

The unit failed to execute when the dependency systemd-sysext.service
was skipped because it's not needed. Normally this is harmless but it
still triggers a OnFailure event as reported in
flatcar/Flatcar#710

Use the same logic for skipping execution as done in
systemd-sysext.service - we can't use "systemctl is-enabled" because
Flatcar's inbuilt unit symlinks are not reported as "enabled".

systemd/system/ensure-sysext.service

@@ -2,6 +2,12 @@
 BindsTo=systemd-sysext.service
 After=systemd-sysext.service
 DefaultDependencies=no
+# Keep in sync with systemd-sysext.service
+ConditionDirectoryNotEmpty=|/etc/extensions
+ConditionDirectoryNotEmpty=|/run/extensions
+ConditionDirectoryNotEmpty=|/var/lib/extensions
+ConditionDirectoryNotEmpty=|/usr/local/lib/extensions
+ConditionDirectoryNotEmpty=|/usr/lib/extensions
 [Service]
 Type=oneshot
 RemainAfterExit=yes