SSO: fix mergeSettings() in case the DB contains empty URLs

[dmihai]

What is this feature?

Previously, the empty URLs from the DB settings were not replaced by the URLs defined in the system settings. This was an issue for providers that require clients to send empty values for auth_url, token_url and api_url. The expectation in that case was that the auth_url, token_url and api_url would get their values from the system settings but that didn't happen if clients sent empty values for those settings. It worked only if clients didn't send them at all.

Why do we need this feature?

This is a fix for the escalation https://github.com/grafana/support-escalations/issues/9668.

Who is this feature for?

For everybody using SSO Settings API.

Which issue(s) does this PR fix?:

Fixes #

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[grafanabot]

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[grafanabot]

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[IevaVasiljeva]

LGTM

[grafana-delivery-bot]

Hello @dmihai!
Backport pull requests need to be either:

• Pull requests which address bugs,
• Urgent fixes which need product approval, in order to get merged,
• Docs changes.

Please, if the current pull request addresses a bug fix, label it with the type/bug label.
If it already has the product approval, please add the product-approved label. For docs changes, please add the type/docs label.
If the pull request modifies CI behaviour, please add the type/ci label.
If none of the above applies, please consider removing the backport label and target the next major/minor release.
Thanks!

[grafana-delivery-bot]

This PR must be merged before a backport PR will be created.