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?

Issue Number: N/A
When a parameter is specified as an array, the type and items of the parameter are not currently rendered in the schema. E.g.

  @ApiProperty({
    name: '_sortBy',
    nullable: true,
  })
  sortBy: string[];

produces

- name: _sortBy
- in: query
- required: true
- schema:
  - nullable: true

This is not valid OpenAPI 3.0.

What is the new behavior?

type and items are specified in the schema.

- name: _sortBy
- in: query
- required: true
- schema:
  - nullable: true
  - type: array
  - items:
    - type: string

This conforms to the OpenAPI 3.0 spec as described here for how arrays should be serialized in the url: https://swagger.io/docs/specification/v3_0/serialization/#serialization-and-rfc-6570

Does this PR introduce a breaking change?

• Yes
• No

This PR would add additional OpenAPI properties an existing spec without any callsite updates. It's possible some existing SDK-generation pipeline based off the specs might be triggered to validate new things, but a spec without items for type: array is not actually valid, which makes me think the chance of breaking SDK generation is fairly slim.

Other information