Fix IAP OAuth credentials not being fetched when IAP is disabled

[sysedwinistrator]

The behavior in validateIAP() of treating .spec.iap.enabled=false as equivalent to .spec.iap=nil and abort all validation is incongruent with that in pkg/backends/features/iap.go, where a full sync of the IAP configuration is triggered as long as .spec.iap!=nil.

The latter behavior is the correct one, as it allows disabling IAP on an ingress where it was previously enabled by setting .spec.spec.enabled=false in the BackendConfig.

However, because pkg/backendconfig/validation.go skips the entire validation and with that the lookup of the OAuth credentials referenced in .spec.iap.oauthclientCredentials.secretName, the desired BackendConfig's OAuth Client ID will be uninitialized and interpreted as empty (""). And in the case that IAP was previously configured with a non-default Client ID, this will trigger a switchingToDefaultError.

TL;DR: This allows disabling IAP on BackendConfig when a non-default OAuth Client has been configured before. It's still necessary to keep the OAuth client credentials in the BackendConfig while IAP is being disabled.

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