how to unset a string field when it's an empty string

For example, b is empty string filed
```
{
a: 'a',
b: ''
}
```

after validating , convert to an object which filed b is deleted
```
{
a: 'a'
}
```

No file, test, or entry point is named in the issue. Start by locating Joi's validation and conversion entry points and reproduce the empty-string case; done means validation returns an object with b omitted while preserving a.

Written by the indexing model from the issue text.