Apply all validation rules to empty strings

[joshdover]

Summary

This fixes a bug with the @kbn/config-schema package where empty strings would skip any other validations defined in the schema.

For example, these examples would not throw a validation error prior to this change:

schema.string({ minLength: 5 }).validate('');

schema.string({
  validate: value => {
    if (!/^[a-z]+$/.test(value)) {
      return 'not all lowercase letters!'
    }
  }
}).validate('');

cc @legrego

Checklist

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• This was checked for cross-browser compatibility, including a check against IE11
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• Unit or functional tests were updated or added to match the most common scenarios
• This was checked for keyboard-only and screenreader accessibility

For maintainers

• This was checked for breaking API changes and was labeled appropriately
• This includes a feature addition or change that requires a release note and was labeled appropriately

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