fix(tree): radial tree with single root doesn't work

[fuchunhui]

Brief Information

This pull request is in the type of:

• bug fixing
• new feature
• others

What does this PR do?

Fixed issues

Details

Before	After

Misc

• The API has been changed (apache/echarts-doc#xxx).
• This PR depends on ZRender changes (ecomfe/zrender#xxx).

Related test cases or examples to use the new APIs

• test/tree-radial.html

Others

Merging options

• Please squash the commits into a single one when merging.

Other information

[echarts-bot]

Thanks for your contribution!
The community will review it ASAP. In the meanwhile, please checkout the coding standard and Wiki about How to make a pull request.

[PhoenixMenia]

thanks for this issue. Working on this and Will be fixed soon.

[echarts-bot]

Congratulations! Your PR has been merged. Thanks for your contribution! 👍

[PhoenixMenia]

这是来自QQ邮箱的假期自动回复邮件。   您好，我最近正在休假中，无法亲自回复您的邮件。我将在假期结束后，尽快给您回复。