内置的class名x6-graph 不自动添加到dom上

### Describe the bug

- react中使用
`container` 添加`className`，渲染出来会自动添加`x6-graph`类名，样式正常生效
但是如果本身不添加`className`，渲染出来就不会有这个类名，导致默认的右键菜单位置不对等问题。

- 相关版本
"@antv/x6": "^2.18.1",
"@antv/x6-plugin-clipboard": "^2.1.6",
"@antv/x6-plugin-dnd": "^2.1.1",
"@antv/x6-plugin-keyboard": "^2.2.3",
"@antv/x6-plugin-scroller": "^2.0.10",
"@antv/x6-plugin-selection": "^2.2.2",
"@antv/x6-react-components": "^2.0.8",
"@antv/x6-react-shape": "^2.2.3",
"react": "^18.2.0",
"react-dom": "^18.2.0"

### Your Example Website or App

无

### Steps to Reproduce the Bug or Issue

无

### Expected behavior

无

### Screenshots or Videos

_No response_

### Platform

- OS: [e.g. macOS, Windows, Linux]
- Browser: [e.g. Chrome, Safari, Firefox]
- Version: [e.g. 2.11.1]

### Additional context

_No response_

The report concerns the React container and its className handling in @antv/x6. Start by reproducing the graph without a supplied className, then inspect the container-rendering entry point and compare it with the className case. Done means the x6-graph class is present in both cases and the default context-menu positioning works.

Written by the indexing model from the issue text.