Change custom_id MISSING comparison to None for Select class to match other components

[krittick]

Summary

This changes the custom_id comparison used in the Select class to match those used in other components, and fixes an issue introduced in #1017 as a result.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, typehinting, examples, ...)