feat: add skip null properties option to partial type

[NovikovEvgeny]

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?

• 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?

PartialType will return a class for which validations are ignored null properties (in addition to undefined properties).

Issue Number: 2843, 2623

What is the new behaviour?

PartialType can optionally return a class for which validations are ignored only for undefined properties, not null properties. This can be useful if you are creating a DTO for a PATCH endpoint: there may be a certain field, say name, that should not be null, but which doesn't need to be included in PATCH updates.

Does this PR introduce a breaking change?

• Yes
• No

Other information

This is actually just a copy-paste of the same PR to mapped-types repo: nestjs/mapped-types#1274
Thanks to @tf3

[podplatnikm]

BUMP

[kamilmysliwiec]

lgtm