[Go] `ai.LookupModel` returns `(*ai.modelActionDef)(nil)` when no model is found, which is not the same as `nil`

[yukinagae]

Describe the bug

The below code does not return nil when no model is found.
Instead of returning nil, it returns (*ai.modelActionDef)(nil) and (*ai.modelActionDef)(nil) == nil always be false.

https://github.com/firebase/genkit/blob/main/go/ai/generate.go#L92-L96

// LookupModel looks up a [Model] registered by [DefineModel].
// It returns nil if the model was not defined.
func LookupModel(provider, name string) Model {
	return (*modelActionDef)(core.LookupActionFor[*GenerateRequest, *GenerateResponse, *GenerateResponseChunk](atype.Model, provider, name))
}

To Reproduce

If I run the sample code below:

	if err := googleai.Init(ctx, nil); err != nil {
		log.Fatal(err)
	}

	genkit.DefineFlow("menuSuggestionFlow", func(ctx context.Context, input string) (string, error) {
		m := googleai.Model("foo")
		if m == nil {
			log.Fatal(" Model not found")
		}
		log.Printf("model: %#v", m)
		log.Printf("model == nil: %#v", m == nil)

It produces the below log where nil check does not work as expected.

time=2024-09-01T11:00:23.509+09:00 level=INFO msg="model: (*ai.modelActionDef)(nil)"
time=2024-09-01T11:00:23.509+09:00 level=INFO msg="model == nil: false"

Expected behavior

ai.LookupModel(provider, name) returns nil when no model is found, as the below ai.LookupEmbedder(provider, name) does.

https://github.com/firebase/genkit/blob/main/go/ai/embedder.go#L68-L76

// LookupEmbedder looks up an [Embedder] registered by [DefineEmbedder].
// It returns nil if the embedder was not defined.
func LookupEmbedder(provider, name string) Embedder {
	action := core.LookupActionFor[*EmbedRequest, *EmbedResponse, struct{}](atype.Embedder, provider, name)
	if action == nil {
		return nil
	}
	return (*embedderActionDef)(action)
}

Runtime (please complete the following information):

• OS: MacOS
• Version: 12.7.6

Go version

• Go: 1.23.0

$ go version
go version go1.23.0 darwin/arm64

Additional context

This blog post Check For Nil Interface In Go explains the tricky aspects of checking for nil interfaces in detail.

[yukinagae]

If I'm on the right track, I'll create a PR for this issue immediately.

[ghchinoy]

Thank you for this!