Fix derived branch name default.

[bosschaert]

Description

The branch name is used by the playwright tests to derive the hostname to use. While the current approach works for PRs, it doesn't work when running the playwright tests on the main branch.

The previous default didn't work if the variable had an empty String as value, in which case it should default to main. There was a default in the code, but this doesn't work in this scenario.

Types of changes

• [X] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [X] I have signed the Adobe Open Source CLA.
• [X] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [X] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [X] All new and existing tests passed.

[aem-code-sync[bot]]

Hello, I'm the AEM Code Sync Bot and I will run some actions to deploy your branch and validate page speed. In case there are problems, just click a checkbox below to rerun the respective action.

• [ ] Re-run PSI checks
• [ ] Re-sync branch

Commits

* [895f8e7](https://github.com/adobe/da-live/commit/895f8e7ca37a0d5c1c625156f3ba3d58d38dd4b7) :white_check_mark: (latest) * [47dae2a](https://github.com/adobe/da-live/commit/47dae2ad1c046840760eb7270fd3fa31579721f8) :white_check_mark: