CustomResource derive injects empty defaults for unset fields

[clux]

Current and expected behavior

A kube derived CRD spec using say;

#[derive(CustomResource, Deserialize, Serialize, Clone, Debug, JsonSchema)]
#[kube(kind = "Document", group = "kube.rs", version = "v1", namespaced)]
#[kube(status = "DocumentStatus")]
pub struct DocumentSpec {
    pub title: String,
}

injects the following .spec properties (outside the schema):

---
apiVersion: apiextensions.k8s.io/v1
kind: CustomResourceDefinition
metadata:
  name: documents.kube.rs
spec:
  group: kube.rs
  names:
    categories: []
    kind: Document
    plural: documents
    shortNames: []
    singular: document
  scope: Namespaced
  versions:
  - additionalPrinterColumns: []
    name: v1
    schema: BIGBLOB_NOT_RELEVANT_HERE

in particular
.spec.names.categories and .spec.names.shortNames and .spec.versions[].additionalPrinterColumn are set even without us explicitly setting the corresponding kube attributes for printcolumn, shortname, nor category.

This is a violation of the spec for CRDs;

• shortnames and categories are both optional on .spec.names
• print columns is optional on .spec.versions[]

Possible solution

Only emit these vectors when they are non-empty. If there are no values in our internal code we have never called any of the attributes and so they should be treated as unset rather than explicit empty.

Additional context

See #1678

Affected crates

kube-derive

Would you like to work on fixing this bug?

maybe

[clux]

A solution here is probably something along the lines of what we do already for selectable fields, inject the surrounding key only if it's non-empty:

kube/kube-derive/src/custom_resource.rs

Lines 509 to 513 in 6a980c6

	let selectable = if !selectable.is_empty() {
	quote! { "selectableFields": fields, }
	} else {
	quote! {}
	};

But also probably needs some improvement in the ::crd fn:

kube/kube-derive/src/custom_resource.rs

Lines 572 to 573 in 6a980c6

	let categories: Vec<String> = #serde_json::from_str(#categories_json).expect("valid categories");
	let shorts : Vec<String> = #serde_json::from_str(#short_json).expect("valid shortnames");