Function optional

optional(propSchema?: boolean | PropSchema): PropSchema
Optional indicates that this model property shouldn't be serialized if it isn't present.

Note that if we use optional together with another prop schema such as custom, the prop schema for custom will be applied first and the result of that serialization will be used to feed into optional. As such, it might be better to just use custom with SKIP to achieve the same goal.
Parameters
- OptionalpropSchema: boolean | PropSchema
  propSchema to (de)serialize the contents of this field
Returns PropSchema
Example
```
createModelSchema(Todo, {
    title: optional(primitive()),
    user: optional(custom(value => value?.name, () => SKIP))
})

serialize(new Todo()) // {}
```
- Defined in types/optional.ts:23