fix: Invalid CRD schema for StackableAffinity contents

[sbernauer]

Description

Fixup of #821

Before

The CustomResourceDefinition "druidclusters.druid.stackable.tech" is invalid: 
* spec.validation.openAPIV3Schema.properties[spec].properties[coordinators].properties[config].properties[affinity].default.podAffinity: Invalid value: "null": podAffinity in body must be of type object: "null"
* spec.validation.openAPIV3Schema.properties[spec].properties[coordinators].properties[config].properties[affinity].default.podAntiAffinity: Invalid value: "null": podAntiAffinity in body must be of type object: "null"
* spec.validation.openAPIV3Schema.properties[spec].properties[coordinators].properties[config].properties[affinity].default.nodeAffinity: Invalid value: "null": nodeAffinity in body must be of type object: "null"
* spec.validation.openAPIV3Schema.properties[spec].properties[historicals].properties[config].properties[affinity].default.nodeAffinity: Invalid value: "null": nodeAffinity in body must be of type object: "null"
* spec.validation.openAPIV3Schema.properties[spec].properties[historicals].properties[config].properties[affinity].default.podAffinity: Invalid value: "null": podAffinity in body must be of type object: "null"
* spec.validation.openAPIV3Schema.properties[spec].properties[historicals].properties[config].properties[affinity].default.podAntiAffinity: Invalid value: "null": podAntiAffinity in body must be of type object: "null"
* spec.validation.openAPIV3Schema.properties[spec].properties[middleManagers].properties[config].properties[affinity].default.podAntiAffinity: Invalid value: "null": podAntiAffinity in body must be of type object: "null"
* spec.validation.openAPIV3Schema.properties[spec].properties[middleManagers].properties[config].properties[affinity].default.nodeAffinity: Invalid value: "null": nodeAffinity in body must be of type object: "null"
* spec.validation.openAPIV3Schema.properties[spec].properties[middleManagers].properties[config].properties[affinity].default.podAffinity: Invalid value: "null": podAffinity in body must be of type object: "null"
* spec.validation.openAPIV3Schema.properties[spec].properties[routers].properties[config].properties[affinity].default.podAffinity: Invalid value: "null": podAffinity in body must be of type object: "null"
* spec.validation.openAPIV3Schema.properties[spec].properties[routers].properties[config].properties[affinity].default.podAntiAffinity: Invalid value: "null": podAntiAffinity in body must be of type object: "null"
* spec.validation.openAPIV3Schema.properties[spec].properties[routers].properties[config].properties[affinity].default.nodeAffinity: Invalid value: "null": nodeAffinity in body must be of type object: "null"
* spec.validation.openAPIV3Schema.properties[spec].properties[brokers].properties[config].properties[affinity].default.nodeAffinity: Invalid value: "null": nodeAffinity in body must be of type object: "null"
* spec.validation.openAPIV3Schema.properties[spec].properties[brokers].properties[config].properties[affinity].default.podAffinity: Invalid value: "null": podAffinity in body must be of type object: "null"
* spec.validation.openAPIV3Schema.properties[spec].properties[brokers].properties[config].properties[affinity].default.podAntiAffinity: Invalid value: "null": podAntiAffinity in body must be of type object: "null"

After

customresourcedefinition.apiextensions.k8s.io/druidclusters.druid.stackable.tech created

Definition of Done Checklist

• Not all of these items are applicable to all PRs, the author should update this template to only leave the boxes in that are relevant
• Please make sure all these things are done and tick the boxes

# Author
- [ ] Changes are OpenShift compatible
- [ ] CRD changes approved
- [ ] Integration tests passed (for non trivial changes)

# Reviewer
- [ ] Code contains useful comments
- [ ] (Integration-)Test cases added
- [ ] Documentation added or updated
- [ ] Changelog updated
- [ ] Cargo.toml only contains references to git tags (not specific commits or branches)

# Acceptance
- [ ] Feature Tracker has been updated
- [ ] Proper release label has been added

[NickLarsenNZ]

LGTM