Update `Embedder` type

[Morgane Dubus]

Description
The Embedder type is currently defined like this:

export type Embedder =
  | OpenAiEmbedder
  | HuggingFaceEmbedder
  | UserProvidedEmbedder

I should accept null too since this is a valid value (we can update the embedders and set one to null when we want to delete it)

Expected behavior
Accept null as a valid value