Previously, attempting to configure the OpenAI provider with a null admin_key would result in an error, making it impossible to use configurations where the admin key might be intentionally unset for certain environments or accounts.

This change:
- Changes the admin_key validation from an error to a warning when not provided
- Only performs the sk-admin- prefix validation when an admin_key is actually provided
- Enables common configurations like:
  ```hcl
  provider openai {
    admin_key = var.openai_api_key # where var.openai_api_key can be null
  }