Fix AutoGen Studio pathPrefix error on Windows

[New-World-2019]

Why are these changes needed?

When I built AutoGen Studio locally in Windows environment, after executing the build command(
gatsby clean && rmdir /s /q ..\autogenstudio\web\ui 2>nul & (set "PREFIX_PATH_VALUE=" || ver>nul) && gatsby build --prefix-paths && xcopy /E /I /Y public ..\autogenstudio\web\ui
), the frontend could not find the page after clicking. After analysis, I found that this was because when the Windows environment variable was set to empty, the process.env.PREFIX_PATH_VALUE value read by the code was undefined, which caused the path problem. So I modified the gatsby-config.ts code and tested it under Windows and Linux without problems.

The change mainly adds an OR operation.When the value of process.env.PREFIX_PATH_VALUE is undefined in Windows, the pathPrefix value is empty. The change can solve the problem that the frontend page cannot be displayed when PREFIX_PATH_VALUE is set to empty in Windows.

Related issue number

Closes #3109

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.