fix(common): right generic type arg constraint for FileValidator#isValid method

[micalevisk]

PR Checklist

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• 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 #12019

What is the new behavior?

the only way to proper narrowing the type of the first arg of FileValidator#isValid method AFIAK is via class generic type arg instead of the method-based one which was introduced in PR #11040

Does this PR introduce a breaking change?

• Yes
• No

Other information

[coveralls]

Pull Request Test Coverage Report for Build 67d4d269-d732-4120-b08b-47d79aa8e1e4

• 3 of 3 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.864%

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Totals
Change from base Build 3dfb6545-1975-4a99-b2ed-aaa49f412a91:	0.0%
Covered Lines:	6364
Relevant Lines:	6853

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💛 - Coveralls

[kamilmysliwiec]

LGTM