Add nullable http proto

[mhoeger]

Adding an alternative to map

resolves Azure/azure-functions-language-worker-protobuf#49

Related to:
Azure/azure-functions-language-worker-protobuf#50
Azure/azure-functions-nodejs-worker#326

Pull request checklist

• My changes do not require documentation changes
  • Otherwise: Documentation issue linked to PR
• My changes should not be added to the release notes for the next release
  • Otherwise: I've added my notes to release_notes.md
• My changes do not need to be backported to a previous version
  • Otherwise: Backport tracked by issue/PR #issue_or_pr
• I have added all required tests (Unit tests, E2E tests)

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