Fix NPE in getDnsFqdns()

[cbosdo]

What does this PR change?

Fix NPE in hardware refresh with a Salt-SSH Centos 7 minion without any activation key

GUI diff

No difference.

• [X] DONE

Documentation

• No documentation needed: only internal and user invisible changes
• [X] DONE

Test coverage

• No tests: already covered

• [X] DONE

Links

Fixes https://bugzilla.suse.com/show_bug.cgi?id=1203611

• [X] DONE

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[mcalmer]

Ok, I will leave this one "open" so the discussion and maybe some changes can continue. For 4.3 I will merge to unblock QE

[lucidd]

Also i just noticed this would not handle the case when the Map does not contain "dns_fqdns". To handle that you can replace the map with .flatMap(s -> Optional.ofNullable(s.get("dns_fqdns")))

[cbosdo]

@lucidd when adding unit tests based on real data I found out that the fix was not completely correct. It should be better (and tested) now