Non-required rules give error on empty parameters

The following sample should pass:

```
const data = {
title: 'This is a title'
}

const rules = {
title: 'required|min:2',
description: 'min:20',
}

validate(data, rules)
```

However the description rule gives an error `{ description: 'The description field must be at least 20 characters.' }`

Start at the validate(data, rules) entry point using the sample in the issue, then trace how the min:20 rule handles the missing description parameter. Done means optional rules allow an absent field while required rules still report missing values, and the sample validation passes without a description error.

Written by the indexing model from the issue text.