feat(mapped-types): add skip null properties option to partial type

[tf3]

With this change, you can create a partial class which does not ignore validations on null properties, but ignores validations on undefined properties. Previously, every class created with PartialType ignored validations on null properties, which may be undesired if you are defining the DTO for a PATCH endpoint. If the option is not defined, the behaviour is unchanged from the previous behaviour.

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).

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 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 feature was discussed here. Note that we will want to make a similar change to the @nestjs/swagger code.

[kamilmysliwiec]

LGTM