ncm-systemd: ensure unit property is defined

[jouvin]

Fixes #1787

[jrha]

Isn't the issue here that the schema marks it as optional, but the validation requires it to be present?

[jrha]

How about jrha@1abc38d instead?

[jouvin]

@jrha I'm happy with your patch too, it should have the same effect for the use case I want to address. I think the main choice criteria between both approaches is whether the configuration module code can deal properly with an undefined unit property. Looking quickly at the code, I have the feeling that the code is not really prepared to deal with no units at all being defined but the actual unit list is built in Systemd/Service.pm by gather_configured_units() which merges systemd units and services defined in ncm-chkconfig so the risk of an empty unit list looks very small and will probably not match a meaningful usecase... So probably it doesn't make a big difference.

[jouvin]

@jrha I replaced my fix by yours. Looks OK. I checked the code in Systemd/Service.pm: gather_configured_units() calls _get_tree() which properly checks the existence of the path so keeping unit undefined should not cause any problem.

[jouvin]

@jrha please, could you merge this PR is you agree with as it would allow to fix errors in several other PRs currently open...

[jrha]

Sorry, was waiting for the tests to finish and got distracted!