Description of problem:

When upgrading the syncthing package (where user services should be restarted after package updates, hence the use of %systemd_user_postun_with_restart), there's a scriptlet error that originates here:

/usr/lib/systemd/systemd-update-helper at line 54

Version-Release number of selected component (if applicable):
systemd-249.4-2.fc35.x86_64

How reproducible:
Always.

Steps to Reproduce:
1. install syncthing (e.g. v1.18.3 from fedora 35 stable repo)
2. enable and start syncthing user session service for some user
2. upgrade syncthing (e.g. to v1.18.4 from fedora 35 koji / updates-testing)
3. observe scriptlet error and not-restarted user units that should have been restarted

Actual results:
User units don't get restarted after the package upgrade.

Expected results:
User units get successfully restarted after package upgrade.

Additional info:

dnf output snippet:

Running transaction
  Preparing        :
  Upgrading        : syncthing-1.18.4-1.fc35.x86_64
  Running scriptlet: syncthing-1.18.4-1.fc35.x86_64
  Running scriptlet: syncthing-1.18.3-1.fc35.x86_64
  Cleanup          : syncthing-1.18.3-1.fc35.x86_64
  Running scriptlet: syncthing-1.18.3-1.fc35.x86_64
/usr/lib/systemd/systemd-update-helper: line 54: unit: unbound variable

  Verifying        : syncthing-1.18.4-1.fc35.x86_64
  Verifying        : syncthing-1.18.3-1.fc35.x86_64

Upgraded:
  syncthing-1.18.4-1.fc35.x86_64