Debian systemd post-install fails due to empty if-clause in template.

[x10an14]

Ref. PR #1498, we've experienced this when utilizing the below flags for fpm:

`fpm` invocation

rm -f ./*.deb

fpm \
    --verbose \
    --output-type deb \
    --input-type dir \
    --name X \
    --version "${1:-Unknown}" \
    --vendor "Y" \
    --maintainer "Y" \
    --description "Lorem Ipsum" \
    --url Z \
    --depends jq \
    --depends sed \
    --depends wireguard \
    --after-install packaging/linux/postinstall \
    --after-upgrade packaging/linux/postinstall \
    --deb-systemd packaging/linux/helper.service \
    bin/linux-client/helper=/usr/sbin/helper \
    bin/linux-client/agent=/usr/bin/X \
    packaging/linux/X.desktop=/usr/share/applications/ \
    packaging/linux/icons/=/usr/share/icons/hicolor/

Package install log

(Reading database ... 646934 files and directories currently installed.)
Preparing to unpack .../X.deb ...
Unpacking X (2021-01-25-f3d0708) over (2021-01-15-b32d8c8) ...
Setting up X (2021-01-25-f3d0708) ...
/var/lib/dpkg/info/X.postinst: 34: Syntax error: "else" unexpected
dpkg: error processing package X (--configure):
 installed X package post-installation script subprocess returned error exit status 2
Processing triggers for mime-support (3.64ubuntu1) ...
Processing triggers for hicolor-icon-theme (0.17-2) ...
Processing triggers for gnome-menus (3.36.0-1ubuntu1) ...
Processing triggers for desktop-file-utils (0.24+linuxmint1) ...
Errors were encountered while processing:
 X
E: Sub-process /usr/bin/dpkg returned an error code (1)

X.postinst, lines 30-36:

systemctl --system daemon-reload >/dev/null || true
debsystemctl=$(command -v deb-systemd-invoke || echo systemctl)
if ! systemctl is-enabled device-agent-helper >/dev/null
then
else
    $debsystemctl restart device-agent-helper >/dev/null || true
fi

And a shellcheck output to confirm that bash/dash/sh can't handle the above if-test:

-> $ shellcheck test.sh
In test.sh line 5:
if ! systemctl is-enabled device-agent-helper >/dev/null
^-- SC1009: The mentioned syntax error was in this if expression.
In test.sh line 6:
then
^-- SC1073: Couldn't parse this then clause. Fix to allow more checks.
In test.sh line 7:
else
^-- SC1048: Can't have empty then clauses (use 'true' as a no-op).
    ^-- SC1072: Unexpected keyword/token. Fix any mentioned problems and try again.
For more information:
  https://www.shellcheck.net/wiki/SC1048 -- Can't have empty then clauses (us...
  https://www.shellcheck.net/wiki/SC1072 -- Unexpected keyword/token. Fix any...
  https://www.shellcheck.net/wiki/SC1073 -- Couldn't parse this then clause. ...

It's obvious to me this can currently happen on latest version (this commit) of templates/deb/postinst_upgrade.sh.erb.
Ref:

if ! systemctl is-enabled <%= service %> >/dev/null 
then
<% if attributes[:deb_systemd_enable?]-%>
    systemctl enable <%= service %> >/dev/null || true
<% end -%>
<% if attributes[:deb_systemd_auto_start?]-%>
    $debsystemctl start <%= service %> >/dev/null || true
<% end -%>
<% if attributes[:deb_systemd_restart_after_upgrade?] -%>
else
    $debsystemctl restart <%= service %> >/dev/null || true
<% end -%>
fi

[x10an14]

Maybe #1750 is related?

[Forage]

Duplicate of #1749