fix(@angular/build): treat empty browsers array as undefined in unit-test builder

[alan-agius4]

Ensure that an empty browsers array is treated as undefined during the option normalization process. This allows the unit-test builder to properly fall back to the Node.js/jsdom environment for Vitest, and allows the Karma runner to use its default configuration.

Closes #32652

[gemini-code-assist]

Code Review

This pull request addresses an issue where an empty browsers array in the unit-test builder options was not handled as an absence of a value. The change correctly treats an empty array as undefined, ensuring the builder falls back to its default behavior. The implementation is sound. I have added one comment pointing out that while this change is a good safeguard, the situation it handles might indicate an underlying issue with schema validation that could be investigated separately.

[alan-agius4]

This PR was merged into the repository. The changes were merged into the following branches:

• main: f17ed30
• 21.2.x: 584f6a2

[anbalase]

@alan-agius4 can this be applied to 20.x as well?

[alan-agius4]

Version 20 is LTS. No new bug fixes, just security fixes.

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