genai: check for non-empty API key (#52)

Fixes #51.

genai/client.go

@@ -55,6 +55,7 @@ func NewClient(ctx context.Context, opts ...option.ClientOption) (*Client, error
 Visit https://ai.google.dev to get one, put it in an environment variable like GEMINI_API_KEY,
 then pass it as an option:
     genai.NewClient(ctx, option.WithAPIKey(os.Getenv("GEMINI_API_KEY")))
+(If you're doing that already, then maybe the environment variable is empty or unset.)
 Import the option package as "google.golang.org/api/option".`)
 	}
 	c, err := gl.NewGenerativeRESTClient(ctx, opts...)
@@ -75,9 +76,9 @@ Import the option package as "google.golang.org/api/option".`)
 // is unexported, and the struct that it populates is in an internal package.
 func hasAPIKey(opts []option.ClientOption) bool {
 	for _, opt := range opts {
-		t := reflect.TypeOf(opt)
-		if t.String() == "option.withAPIKey" {
-			return true
+		v := reflect.ValueOf(opt)
+		if v.Type().String() == "option.withAPIKey" {
+			return v.String() != ""
 		}
 	}
 	return false

genai/client_test.go

@@ -513,4 +513,8 @@ func TestNoAPIKey(t *testing.T) {
 	if err == nil {
 		t.Fatal("got nil, want error")
 	}
+	_, err = NewClient(context.Background(), option.WithAPIKey(""))
+	if err == nil {
+		t.Fatal("got nil, want error")
+	}
 }