fix: fixes the infer bug of ConfigService.get

[JasonHK]

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)
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PR Type

What kind of change does this PR introduce?

[x] 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: N/A

When calling ConfigService.get(propertyPath, defaultValue), TypeScript will incorrectly infered the type of defaultValue as the return type if the generic was omitted.

const value: string = this.configService.get('database.host', 'localhost');

Will be infered as:

const value: string = this.configService.get<'localhost'>('database.host', 'localhost');

What is the new behavior?

const value: string = this.configService.get('database.host', 'localhost');

Will be infered as:

const value: string = this.configService.get<string>('database.host', 'localhost');

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[kamilmysliwiec]

Thank you!