Allow nulls for helm values

[makhov]

Description

While we ​​clean up the values we convert null values to strings, which breaks some use cases. For instance:

apiVersion: k0s.k0sproject.io/v1beta1
kind: ClusterConfig
metadata:
  name: k0s
spec:
  extensions:
    helm:
      repositories:
      - name: aws-cloud-controller-manager
        url: https://kubernetes.github.io/cloud-provider-aws
      charts:
      - name: aws-cloud-controller-manager
        namespace: kube-system
        chartname: aws-cloud-controller-manager/aws-cloud-controller-manager
        version: "0.0.8"
        values: |
          nodeSelector: null
          args:
           - --v=2

causes the error:

* DaemonSet in version "v1" cannot be handled as a DaemonSet: json: cannot unmarshal string into Go struct field PodSpec.spec.template.spec.nodeSelector of type map[string]string

but it works perfectly with the helm cli.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation update

How Has This Been Tested?

• [x] Manual test
• [ ] Auto test added

Checklist:

• [x] My code follows the style guidelines of this project
• [x] My commit messages are signed-off
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules
• [x] I have checked my code and corrected any misspellings

[k0s-bot]

Successfully created backport PR for release-1.28:

• 4667

[k0s-bot]

Successfully created backport PR for release-1.29:

• 4668

[k0s-bot]

Successfully created backport PR for release-1.30:

• 4669