xsml中的空style节点，导致页面报错

示例代码：

```xsml
<xsml version="1.0">
  <head>
    <title>JSAR Widget</title>
    <link id="model" rel="mesh" type="octstream/glb" href="./model/yellow_duck.glb" />
    <script src="./lib/main.ts"></script>
    <style>

    </style>
  </head>
  <space>
    <mesh id="model" ref="model" selector="__root__" />
  </space>
</xsml>
``` 