feat: implement `undefinedAsNull` keyword for `enum` type

[alexander-akait]

This PR contains a:

• [ ] bugfix
• [x] new feature
• [ ] code refactor
• [x] test update
• [ ] typo fix
• [ ] metadata update

Motivation / Use-Case

part of https://github.com/webpack/webpack/pull/17339/

Breaking Changes

No

Additional Info

Only for enum

We need backport it for v3 too Also we need to implement this for tooling

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.04 :tada:

Comparison is base (6d5a65f) 97.43% compared to head (7c5a40e) 97.48%.

:exclamation: Current head 7c5a40e differs from pull request most recent head 10443c3. Consider uploading reports for the commit 10443c3 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #175 +/- ## ========================================== + Coverage 97.43% 97.48% +0.04% ========================================== Files 6 7 +1 Lines 741 755 +14 Branches 318 321 +3 ========================================== + Hits 722 736 +14 Misses 17 17 Partials 2 2 ``` | [Impacted Files](https://app.codecov.io/gh/webpack/schema-utils/pull/175?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webpack) | Coverage Δ | | |---|---|---| | [src/ValidationError.js](https://app.codecov.io/gh/webpack/schema-utils/pull/175?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webpack#diff-c3JjL1ZhbGlkYXRpb25FcnJvci5qcw==) | `96.52% <100.00%> (+0.02%)` | :arrow_up: | | [src/keywords/undefinedAsNull.js](https://app.codecov.io/gh/webpack/schema-utils/pull/175?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webpack#diff-c3JjL2tleXdvcmRzL3VuZGVmaW5lZEFzTnVsbC5qcw==) | `100.00% <100.00%> (ø)` | | | [src/validate.js](https://app.codecov.io/gh/webpack/schema-utils/pull/175?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=webpack#diff-c3JjL3ZhbGlkYXRlLmpz) | `100.00% <100.00%> (ø)` | |

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