fix(next-drupal): undefined is not serializable

[yobottehg]

This pull request is for: (mark with an "x")

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

GitHub Issue: #793

• [ ] I need help adding tests. (mark with an "x") Code changes need test coverage. If you don't know how to make tests, check this box to ask for help.

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