Fix minimal bundled build

[compulim]

Fixes #5518.

Changelog Entry

Fixed

• Fixed #5518. Minimal bundled build should work properly, in PR #5507, by @compulim

Description

HTMLContentTransformerComposer did not properly check for undefined.

We should enable @tsconfig/strictest to catch these issues sooner.

Specific Changes

• Updates HTMLContentTransformerComposer to coalesce undefined with empty array

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)