fix(litellm): fix default value for api-key field
What does this merge request do and why?
Within feat: allow passing LiteLLM model API key (!821 - merged) it became possible to specify api-key to litellm models. When api-key field is not specified, the stubbed value should be used: it's required field for litellm, but a model can be potentially deployed without the authorization, so the api-key is optional.
This MR handles the case when api-key field is explicitly specified as None. We still need to use the stubbed value there.