Server - Add an option to always expose empty groups in GetCapabilities

[Gustry]

Since #51724 empty groups are not exposed in the GetCapabilities.
Some groups can be empty after a server plugin for instance hiding layers.

But for some projects, the GIS client still wants all groups to be fetched from the server. The PR #51724 is a choice but doesn't fit for all needs.

I propose to add a flag if we want the group to be always exposed in the GetCapabilities, even if it's empty.

Can this be backported to the current LTR 3.34 ? (or at least the server part if possible). #51724 is breaking a change for people migrating from 3.28 to the new LTR.

CC @pblottiere @elpaso @rldhont

[github-actions]

🪟 Windows builds ready!

Windows builds of this PR are available for testing here. Debug symbols for this build are available here.

(Built from commit f2110df)

[elpaso]

I don't see the use case for that and I'd prefer not to add another option.
Can you elaborate why this is necessary (when do you want to see empty groups)?
If - as you said - the content of the group was hidden by a plugin cannot you just rewrite the plugin to not empty the group?

[Gustry]

I understand @elpaso.
I got another way around.