Strings.ToRawInfo generate empty node

[Encorpluptit]

Hello,

Issue

The ToRawInfo method on openapiv3.Strings return an empty node without writing actual content.

                    type: array
                    items:
                        discriminator:
                            propertyName: type
                            mapping: {}  # <------ Empty node
                        anyOf:
                            - $ref: '...'

---

Current implementation:

gnostic-models/openapiv3/OpenAPIv3.go

Lines 8557 to 8565 in 289d7b4

	// ToRawInfo returns a description of Strings suitable for JSON or YAML export.
	func (m *Strings) ToRawInfo() *yaml.Node {
	info := compiler.NewMappingNode()
	if m == nil {
	return info
	}
	// &{Name:additionalProperties Type:NamedString StringEnumValues:[] MapType:string Repeated:true Pattern: Implicit:true Description:}
	return info
	}

---

Fix:

// ToRawInfo returns a description of Strings suitable for JSON or YAML export.
func (m *Strings) ToRawInfo() *yaml.Node {
	info := compiler.NewMappingNode()
	if m == nil {
		return info
	}
	// &{Name:additionalProperties Type:NamedString StringEnumValues:[] MapType:string Repeated:true Pattern: Implicit:true Description:}
	for _, p := range m.AdditionalProperties {
		info.Content = append(info.Content, compiler.NewScalarNodeForString(p.Name))
		info.Content = append(info.Content, compiler.NewScalarNodeForString(p.Value))
	}
	return info
}

Produces:

                        discriminator:
                            propertyName: type
                            mapping:
                                name_example: 'value_example'
                                # ...
                        anyOf:
                            - $ref: '...'

---

Made this PR to fix it

Hope you see this soon !