refactor(utils): generate unique id only when needed

[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?

uuidv4() is being called even if it's not used.

What is the new behavior?

Inlines the uuid() call to rely on JS short circuit on OR operation to generate the uuid v4 only if the factory key was not provided. The factory.KEY will always be defined for those that are using registerAs, thus they can avoid the computation of uuidv4()

$ node
> const foo = () => {}
> const bar = () => console.log('called')
> false || foo( bar() )
called
undefined
> true || foo( bar() )
true

Does this PR introduce a breaking change?

• Yes
• No

[kamilmysliwiec]

Nice! Thank you