resolveCSSVariables: Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'indexOf')

https://github.com/doocs/md/issues/1126

Repro: [test.ts](https://github.com/user-attachments/files/23382091/test.ts)

It should set `resolveCSSVariables` to `false`，then run success.

I think we can remove this feature and let user use [postcss-custom-properties](https://github.com/csstools/postcss-plugins/tree/main/plugins/postcss-custom-properties) to handle css variables.

https://github.com/doocs/md/pull/1131

Start by running the attached test.ts reproduction with resolveCSSVariables set to false, then inspect the option's handling in Juice and the linked pull request 1131. Done means the reproduction completes successfully without the uncaught TypeError and the intended CSS-variable behavior is clear.

Written by the indexing model from the issue text.