dynamic resource allocation: avoid nil vs. empty ambiguity in parameter references

Previously, empty string and nil both meant the same thing (core API group). We
don't need two different ways of expressing that.

Author: pohly

keps/sig-node/3063-dynamic-resource-allocation/README.md

@@ -1529,10 +1529,10 @@ type ResourceClaimTemplate struct {
 // parameters for a ResourceClass.
 type ResourceClassParametersReference struct {
 	// APIGroup is the group for the resource being referenced.
-	// If APIGroup is not specified, the specified Kind must be in the core API group.
+	// If APIGroup is empty, the specified Kind must be in the core API group.
 	// For any other third-party types, APIGroup is required.
 	// +optional
-	APIGroup *string
+	APIGroup string
 	// Kind is the type of resource being referenced
 	Kind string
 	// Name is the name of resource being referenced
@@ -1548,10 +1548,10 @@ type ResourceClassParametersReference struct {
 // as the ResourceClaim.
 type ResourceClaimParametersReference struct {
 	// APIGroup is the group for the resource being referenced.
-	// If APIGroup is not specified, the specified Kind must be in the core API group.
+	// If APIGroup is empty, the specified Kind must be in the core API group.
 	// For any other third-party types, APIGroup is required.
 	// +optional
-	APIGroup *string
+	APIGroup string
 	// Kind is the type of resource being referenced
 	Kind string
 	// Name is the name of resource being referenced