Doesn't read from stdin when passing "`-`"

[Tachi107]

opensysusers doesn't read the configuration from stdin when invoked as

systemd-sysusers -

Which can be handy in tests.

[cromerc]

https://www.freedesktop.org/software/systemd/man/latest/sysusers.d.html#

This is an implementation detail of the program "systemd-sysusers". However opensysusers was written to follow the opensysusers.d spec which makes no mention of this at all.

Should the spec be followed? Or should this be a drop in replacement for systemd-sysusers?