fix: add fallback to providerLabel (#3999)

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 "test@test.com" 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 "test@test.com" at provider "google". If you do not wish to link that account, please start a new login flow.`

selfservice/strategy/oidc/strategy.go

@@ -642,6 +642,9 @@ func (s *Strategy) handleError(w http.ResponseWriter, r *http.Request, f flow.Fl
  provider, _ := s.provider(r.Context(), r, providerID)
  if provider != nil && provider.Config() != nil {
  providerLabel = provider.Config().Label
+ if providerLabel == "" {
+ providerLabel = provider.Config().Provider
+ }
  }
  lf.UI.Messages.Add(text.NewInfoLoginLinkMessage(dc.DuplicateIdentifier, providerLabel, newLoginURL))