fix(next-drupal): undefined is not serializable

[rainer-friederich]

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• examples/*
• modules/next
• packages/next-drupal
• starters/basic-starter
• starters/graphql-starter
• starters/pages-starter
• Other

GitHub Issue: #793

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Describe your changes

When serializing the menu props in getStaticProps in Next.js 15 pages router it fails because undefined is not serializable. This changes undefined to null so it can be serialized.

The same issue should happen with app router and the export target

[vercel]

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