Strip nullable for Server-side apply

staging/src/k8s.io/apiextensions-apiserver/pkg/apiserver/customresource_handler.go

@@ -1245,7 +1245,7 @@ func buildOpenAPIModelsForApply(staticOpenAPISpec *spec.Swagger, crd *apiextensi
 
 	specs := []*spec.Swagger{}
 	for _, v := range crd.Spec.Versions {
-		s, err := builder.BuildSwagger(crd, v.Name, builder.Options{V2: false, StripDefaults: true, StripValueValidation: true, AllowNonStructural: true})
+		s, err := builder.BuildSwagger(crd, v.Name, builder.Options{V2: false, StripDefaults: true, StripValueValidation: true, StripNullable: true, AllowNonStructural: true})
 		if err != nil {
 			return nil, err
 		}

staging/src/k8s.io/apiextensions-apiserver/pkg/apiserver/schema/skeleton.go

@@ -49,3 +49,17 @@ func (s *Structural) StripValueValidations() *Structural {
 	v.Visit(s)
 	return s
 }
+
+// StripNullable returns a copy without nullable.
+func (s *Structural) StripNullable() *Structural {
+	s = s.DeepCopy()
+	v := Visitor{
+		Structural: func(s *Structural) bool {
+			changed := s.Nullable
+			s.Nullable = false
+			return changed
+		},
+	}
+	v.Visit(s)
+	return s
+}

staging/src/k8s.io/apiextensions-apiserver/pkg/controller/openapi/builder/builder.go

@@ -76,6 +76,9 @@ type Options struct {
 	// Strip value validation.
 	StripValueValidation bool
 
+	// Strip nullable.
+	StripNullable bool
+
 	// AllowNonStructural indicates swagger should be built for a schema that fits into the structural type but does not meet all structural invariants
 	AllowNonStructural bool
 }
@@ -101,6 +104,9 @@ func BuildSwagger(crd *apiextensions.CustomResourceDefinition, version string, o
 					if opts.StripValueValidation {
 						schema = schema.StripValueValidations()
 					}
+					if opts.StripNullable {
+						schema = schema.StripNullable()
+					}
 
 					schema = schema.Unfold()
 				}

test/integration/apiserver/apply/apply_crd_test.go

@@ -190,7 +190,8 @@ func TestApplyCRDStructuralSchema(t *testing.T) {
 										"type": "string"
 									},
 									"protocol": {
-										"type": "string"
+										"type": "string",
+										"nullable": true
 									}
 								},
 								"required": [