Fix ID field to be optional (#911)

The ID field is not always present for streaming responses. Without omitempty, the entire ToolCall struct will be missing.

chat.go

@@ -179,7 +179,7 @@ func (m *ChatCompletionMessage) UnmarshalJSON(bs []byte) error {
 type ToolCall struct {
 	// Index is not nil only in chat completion chunk object
 	Index    *int         `json:"index,omitempty"`
-	ID       string       `json:"id"`
+	ID       string       `json:"id,omitempty"`
 	Type     ToolType     `json:"type"`
 	Function FunctionCall `json:"function"`
 }