fix: Validation returns an `unknownValue` error for strings when `forbidUnknownValues: false`

[aklemen]

Description

Even though forbidUnknownValues: false is set on the validation options, the validation of string values still returns an unknownValue error.

Minimal code-snippet showcasing the problem

const someString = 'test';
const validationErrors = validateSync(someString, { forbidUnknownValues: false });

Expected behavior

validationErrors should be

[] // no errors

Actual behavior

validationErrors is

[{"children": [], "constraints": {"unknownValue": "an unknown value was passed to the validate function"}, "property": undefined, "target": {"forbidUnknownValues": false}, "value": undefined}]