refactor: more concise typings for isDefined utility function

[reybok]

Description

When using the utility function isDefined, typescript can properly ignore undefined and null with stricter typing.

Example

function example(param?: string) {
  if (!isDefined(param)) return
  console.log(param.charAt(0)) // Typescript now knows that param _must_ be a string and does not throw a warning param might be undefined
}

Checklist

• the pull request title describes what this PR does (not a vague title like Update index.md)
• the pull request targets the default branch of the repository (develop)
• the code follows the established code style of the repository
  • npm run prettier:check passes
  • npm run lint:check passes
• tests are added for the changes I made (if any source code was modified → it wasn't )
• documentation added or updated ← stays the same
• I have run the project locally and verified that there are no errors

Fixes

This change is so simple, I hoped we don't need to clutter the issue tracker. If you want me to open a feature request, I'll gladly do so.

[reybok]

Dear Maintainers,

let me preface this with a big thank you for this awesome library that I use excessively in my NestJS server 👍

Just letting you know that I won't be available until 1st of August but I still want to see this PR through and love to contribute in any helpful way once I'm back.

Thanks in advance ☀️

[braaar]

Better typings are always great to see. Thanks!

[braaar]

Keep in mind that this is an internal validation function only used by the IsDefined validator, so this doesn't really affect usage of the library.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.