We use the phrase "unpopulated" in systemd.unit(5) too.
And "/etc/ is empty" is simply spurious.
<para><command>systemd-firstboot</command> initializes basic system settings interactively during the
first boot, or non-interactively on an offline system image. The service is started during boot if
<varname>ConditionFirstBoot=yes</varname> is met, which essentially means that <filename>/etc/</filename>
- is empty, see
+ is unpopulated, see
<citerefentry><refentrytitle>systemd.unit</refentrytitle><manvolnum>5</manvolnum></citerefentry> for
details.</para>