fix: add fallback to providerLabel

[Pedr0Rocha]

This adds a fallback to the provider label when trying to register a duplicate identifier with an oidc.

Current error message:

Signing in will link your account to "[email protected]" at provider "". If you do not wish to link that account, please start a new login flow.

The label represents an optional label for the UI, but in my case it's always empty. I suggest we fallback to the provider when the label is not present. In case the label is present, the behaviour won't change.

Fallback to provider:

Signing in will link your account to "[email protected]" at provider "google". If you do not wish to link that account, please start a new login flow.

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got the approval (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

[CLAassistant]

All committers have signed the CLA.