Add config flag to keep empty groups

[marcojarjour]

• Add config flag to keep empty groups

Ref: #833

[blizzz]

Hey @marcojarjour,

thank you for this contribution!

Generally this is an improvement we surely can have.

I'd like to ask you to switch the config away from config.php and turn it into an app-config via $this->config->getAppValue().

One word on whether to have this setting IdP-dependent or as a general flag, like you implemented it. The first one allows more fine-grained control, but I am doubtful this is necessary. So going with the global approach is fine.

You may consider adding a global configuration knob for this as well, but it would regard it optional for acceptance. It's ok to have it as a hidden flag in the beginning.

In the ideal case, equip this with an integration test. While preferred, I would take it also without one since the change is simple and not dangerous. It's just that code may change over time and having one makes it more robust.

[blizzz]

P.S.: and please sign your commit to let the DCO test pass, cf. https://github.com/nextcloud/user_saml/pull/911/checks?check_run_id=33591334717

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