Add syntax sugar as inverted exists? method

[loqimean]

Motivation / Background

This Pull Request has been created because we have so many syntax sugar, like for present? method we have blank?, and so on, so, that's would be really nice to add opposite method for a exists? instead of writing always !, or using unless

Detail

This Pull Request changes adds just an inverted exists? method called missing?. So, it's really simple, but powerful change

Checklist

Before submitting the PR make sure the following are checked:

• [x] This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• [x] Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• [ ] Tests are added or updated if you fix a bug or add a feature.
• [ ] CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.