fix: return always data property in response

internal/handler/response.go

@@ -9,7 +9,7 @@ type APIResponse struct {
 	// Indicates if the request was successful
 	Success bool `json:"success"`
 	// The response data
-	Data any `json:"data,omitempty"`
+	Data any `json:"data"`
 	// List of errors if any occurred
 	Errors []APIError `json:"errors,omitempty"`
 	// Optional message