literal strings as non-empty-string

Should literal strings be a `non-empty-string`?

The problem I occurred is the following:

```php
/**
 * @return list<non-empty-string>
 */
function a(): array
{
    // Imaging I have an external declared type like this:
    /** @var array{foo?: list<non-empty-string>} $array */
    $array = [];

    return $array['foo'] ?? ['bar'];
}
```

https://psalm.dev/r/0cb0b9aa53