fix(common): type def for cors delegated callback function option

[micalevisk]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: closes #12816

What is the new behavior?

we can now supply null to the callback as the cors docs suggests

A better definition would be:

export interface CorsOptionsCallback {
  (error: Error): void;
  (error: null, options: CorsOptions): void;
}

but I believe this would introduce a breaking change

Does this PR introduce a breaking change?

• Yes
• No

Other information

[coveralls]

Pull Request Test Coverage Report for Build defe7298-93b2-4ba1-87d9-404616e425af

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.258%

---

Totals
Change from base Build 28622249-a4a3-41da-b843-b4a9abe03767:	0.0%
Covered Lines:	6697
Relevant Lines:	7259

---

💛 - Coveralls

[kamilmysliwiec]

lgtm