feat(@nestjs/graphql): add support for skipNullProperties in PartialType

[bloom-alex]

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?

Adding the same skipNullProperties as this PR https://github.com/nestjs/swagger/pull/2863/files added for the Swagger version of PartialType. Original feature coming from this Mapped-Type PR nestjs/mapped-types#1274

Issue Number: N/A

What is the new behavior?

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 mutation endpoint: there may be a certain field, say name, that should not be null, but which doesn't need to be included in updates.

Does this PR introduce a breaking change?

• Yes
• No

Other information