fix(core/select-item): check by undefined not by value

[jul-lam]

💡 What is the current behavior?

0 is currently not a valid value option for the ix-select-item.

GitHub Issue Number: xxx

🆕 What is the new behavior?

0 is now a valid value option for the ix-select-item.

🏁 Checklist

A pull request can only be merged if all of these conditions are met (where applicable):

• 🦮 Accessibility (a11y) features were implemented
• 🗺️ Internationalization (i18n) - no hard coded strings
• 📲 Responsiveness - components handle viewport changes and content overflow gracefully
• 📄 Documentation was reviewed/updated (pnpm run docs)
• 🧪 Unit tests were added/updated and pass (pnpm test)
• 📸 Visual regression tests were added/updated and pass (Guide)
• 🧐 Static code analysis passes (pnpm lint)
• 🏗️ Successful compilation (pnpm build, changes pushed)

👨‍💻 Help & support

The test is skipped, since html only uses string values, which the value prop also will use in 3.0.

[changeset-bot]

🦋 Changeset detected

Latest commit: ae95b85

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@siemens/ix	Patch
@siemens/ix-angular	Patch
@siemens/ix-react	Patch
@siemens/ix-vue	Patch

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[nuke-ellington]

Do we have a test case?

[jul-lam]

@nuke-ellington - I haven’t set up a test because HTML exclusively uses string values for properties. We could however set up a test using React, but I was not sure if that’s needed, since our value property will take exclusively string in our next major release.