fix(model): remove unnecessary conversion of undefined -> null in findById

[vkarpov15]

Fix #15551

Summary

The findById() documentation indicates that findOne({ _id: undefined }) will return an arbitrary document, which is no longer accurate. I also removed the undefined check in findById(), which is no longer necessary because findOne({ _id: undefined }) no longer returns an arbitrary document.

Examples

[Copilot]

Pull Request Overview

This PR removes unnecessary undefined handling in the findById() method and updates its documentation to reflect current behavior. The change eliminates a conversion that was previously needed when findOne({ _id: undefined }) returned arbitrary documents, which is no longer the case.

• Remove conditional check that converts undefined to null in findById()
• Update documentation to reflect that findById(id) is now equivalent to findOne({ _id: id })
• Remove outdated documentation explaining the difference in undefined handling between the two methods