Fix validation for Request parameter that is an array without default value

If Request parameter is an array without default value and parameter is not set - then an error should be thrown that "parameter value is not an array".

Before this fix - even if default value is not specified, then in this line:

```
if ($config->array) {
    $default = (array) $default;
}
```

It is converted to an empty array. 
So even if Request parameter is missing - no error is thrown but instead empty array is assigned to the parameter.