Check for an empty value in validateField

reflect.TypeOf() can take a nil (it then returns a nil), but
Kind() panics on a nil.

Fixes #20627

pkg/api/validation/schema.go

@@ -297,6 +297,10 @@ func (s *SwaggerSchema) isGenericArray(p swagger.ModelProperty) bool {
 var versionRegexp = regexp.MustCompile(`^v.+\..*`)
 
 func (s *SwaggerSchema) validateField(value interface{}, fieldName, fieldType string, fieldDetails *swagger.ModelProperty) []error {
+	allErrs := []error{}
+	if reflect.TypeOf(value) == nil {
+		return append(allErrs, fmt.Errorf("unexpected nil value for field %v", fieldName))
+	}
 	// TODO: caesarxuchao: because we have multiple group/versions and objects
 	// may reference objects in other group, the commented out way of checking
 	// if a filedType is a type defined by us is outdated. We use a hacky way
@@ -310,7 +314,6 @@ func (s *SwaggerSchema) validateField(value interface{}, fieldName, fieldType st
 		// if strings.HasPrefix(fieldType, apiVersion) {
 		return s.ValidateObject(value, fieldName, fieldType)
 	}
-	allErrs := []error{}
 	switch fieldType {
 	case "string":
 		// Be loose about what we accept for 'string' since we use IntOrString in a couple of places