gpu-operator Fix chart's devicePlugin.config fields evaluated to objects instead of strings when empty

Since these fields are not quoted when templated, using any special value such as null, or an empty string may result in them being evaluated as objects.

This causes an error when evaluating against the CRD definition while applying the object. As there may be more fields with the same behavior, I would probably expand this PR if needed.

While I agree that these fields shouldn't be empty, not quoting those values anyway may lead to issues and misunderstandings.

Testing process

Create a test values file with an empty value for the config name, and a special name.

devicePlugin:
  config:
    name: "null"
    create: true
    default: ""

Pre-fix:

Observe that the devicePlugin.config fields are not formatted correctly

 nefast@sapphire $ helm template nvidia-gpu-operator . -f test.yaml | grep devicePlugin -A 10
  devicePlugin:
    enabled: true
    repository: nvcr.io/nvidia
    image: k8s-device-plugin
    version: "v0.18.1"
    imagePullPolicy: IfNotPresent
    config:
      name: null
      default:

Post-fix:

Observe that the devicePlugin.config fields are templated properly and that it passes server validation

nefast@sapphire $ helm template nvidia-gpu-operator . -f test.yaml | grep devicePlugin -A 8
  devicePlugin:
    enabled: true
    repository: nvcr.io/nvidia
    image: k8s-device-plugin
    version: "v0.18.1"
    imagePullPolicy: IfNotPresent
    config:
      name: "null"
      default: ""

Closes #1993