fix(litellm): guard args map against nil arguments

pkg/litellm/model.go

@@ -233,6 +233,9 @@ func oaiChoiceToContent(choice oaiChoice) *genai.Content {
 	for _, tc := range msg.ToolCalls {
 		var args map[string]any
 		_ = json.Unmarshal([]byte(tc.Function.Arguments), &args)
+		if args == nil {
+			args = map[string]any{}
+		}
 		parts = append(parts, &genai.Part{
 			FunctionCall: &genai.FunctionCall{
 				ID:   tc.ID,