fix(charts/linkwarden): adding existingSecret in auth.sso list does not prevent new secret from being created

[helionmusic]

What this PR does / why we need it

Currently in the Linkwarden chart, when setting .Values.linkwarden.auth.sso[*].existingSecret, the chart still attempts to create the auth secret for that given provider. Beyond that there is an error when clientId and clientSecret are not set, leading to your values file requiring at least an empty value for both when using an existing secret.

Special notes for your reviewer

We could probably bundle this in with #33 - I've left the Chart version untouched for that reason. If you'd rather this be a separate version bump I can add the change

Checklist

• [ ] Chart Version bumped
• [ ] User name added to AUTHORS file
• [x] Title of the PR starts with a valid commit scope as detailed in the CONTRIBUTING (e.g. fix(charts/linkwarden): ...)

[github-actions[bot]]

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