RayCluster Helm: Make volumeMounts and volumes optional for workers

[calizarr]

Add defaults to the above and merge otherwise

Why are these changes needed?

The current Helm chart errors out inscrutably as described here

Related issue number

Closes #1668

Checks

It's a Helm chart so I did manual testing on outputs etc.

To test, create the below file as extra-values.yaml and pass it as an argument to helm via -f:

additionalWorkerGroups:
  testGroup:
    disabled: false
    minReplicas: 0
    maxReplicas: 1
    replicas: 1

• I've made sure the tests are passing.
• Testing Strategy
  • Unit tests
  • Manual tests
  • This PR is not tested :(

[kevin85421]

Thanks!