Treat null value as any type when generating schema from Helm chart

[mamachanko]

What this PR does / why we need it:

This change treats null values in Helm chart values as any type using a type union of all possible types being nullable. That is because the expected type cannot be inferred from null.

When releasing a Package from a Helm chart that has null values, the result looks like this:

apiVersion: data.packaging.carvel.dev/v1alpha1
kind: Package
metadata:
  creationTimestamp: null
  name: test.mamachanko.com.0.0.0+build.1737555606
spec:
  refName: test.mamachanko.com
  releasedAt: "2025-01-22T14:20:11Z"
  template:
    spec:
      deploy:
      - kapp: {}
      fetch:
      - imgpkgBundle:
          image: index.docker.io/mamachanko/helm-test@sha256:f80aac3740c96da062f922790482c3aebe7c2c57061bdf0b4208b4833e276ac9
      template:
      - helmTemplate:
          path: helm
      - ytt:
          paths:
          - '-'
      - kbld:
          paths:
          - '-'
          - .imgpkg/images.yml
  valuesSchema:
    openAPIv3:
      properties:
        credentials:
          description: helm/values.yaml
          properties:
            password:
              oneOf:
              - default: null
                nullable: true
                type: integer
              - default: null
                nullable: true
                type: number
              - default: null
                nullable: true
                type: boolean
              - default: null
                nullable: true
                type: string
              - default: null
                nullable: true
                type: object
              - default: null
                items: {}
                nullable: true
                type: array
            username:
              oneOf:
              - default: null
                nullable: true
                type: integer
              - default: null
                nullable: true
                type: number
              - default: null
                nullable: true
                type: boolean
              - default: null
                nullable: true
                type: string
              - default: null
                nullable: true
                type: object
              - default: null
                items: {}
                nullable: true
                type: array
          type: object
      type: object
  version: 0.0.0+build.1737555606

as opposed to the previous:

apiVersion: data.packaging.carvel.dev/v1alpha1
kind: Package
metadata:
  creationTimestamp: null
  name: test.mamachanko.com.0.0.0+build.1737555606
spec:
  refName: test.mamachanko.com
  releasedAt: "2025-01-22T14:20:11Z"
  template:
    spec:
      deploy:
      - kapp: {}
      fetch:
      - imgpkgBundle:
          image: index.docker.io/mamachanko/helm-test@sha256:f80aac3740c96da062f922790482c3aebe7c2c57061bdf0b4208b4833e276ac9
      template:
      - helmTemplate:
          path: helm
      - ytt:
          paths:
          - '-'
      - kbld:
          paths:
          - '-'
          - .imgpkg/images.yml
  valuesSchema:
    openAPIv3:
      properties:
        credentials:
          description: helm/values.yaml
          properties:
            password:
              default: "null"
              type: "null"
            username:
              default: "null"
              type: "null"
          type: object
      type: object
  version: 0.0.0+build.1737555606

Which issue(s) this PR fixes:

Fixes #1630

Does this PR introduce a user-facing change?

fix: treat `null` in Helm value as _any_ when generating OpenAPI v3 schema

Additional Notes for your reviewer:

Review Checklist:

• Follows the developer guidelines
• [ x Relevant tests are added or updated
• Relevant docs in this repo added or updated
• Relevant carvel.dev docs added or updated in a separate PR and there's
  a link to that PR
• Code is at least as readable and maintainable as it was before this
  change

Additional documentation e.g., Proposal, usage docs, etc.:

[nebhale]

Nobody asked me, but I love it!

[mamachanko]

whoops. failing e2e tests. i'm on it ...

[mamachanko]

whoops. failing e2e tests. i'm on it ...

green! ✅

[100mik]

Thanks a lot for the PR! Looks good 🚀