(JavaScript/TypeScript target) Runtime parsing assigns `undefined` values

When targeting ts/js from JSON schema runtime parser assigns `undefiend` values if value isn't present in input and wasn't required in schema. I found this behaviour very annoying, because it would just spam console with `undefined` values.

Here is example (if needed):
```json
{
"$schema": "http://json-schema.org/draft-07/schema",
"title": "demo",
"type": "object",
"properties": {
"name": {
"type": "string"
}
}
}
```
```console
quicktype schema.json -s schema -o out.js
```
```js
// @filename main.js
import { toOut } from './out.js'

console.log(toOut('{}'))
//=> { name: undefined }
```

I think we can probably simplify generated code for runtime parsing.