[NFR]When not found option renturn null

```php
		$this->validate(new StringLengthValidator(array(
			'max' => 20,
			'min' => 1,
			'messageMaximum' => 'messageMaximum',
			'messageMinimum' => 'messageMinimum'
		)));
```
That must be throw error
```c
	PHALCON_INIT_VAR(field);
	phalcon_call_method_p1(field, this_ptr, "getoption", option);
	if (Z_TYPE_P(field) != IS_STRING) {
		PHALCON_THROW_EXCEPTION_STR(phalcon_mvc_model_exception_ce, "Field name must be a string");
		return;
	}
```
```c
PHP_METHOD(Phalcon_Mvc_Model_Validator, getOption){
	// ```
	RETURN_MM_EMPTY_STRING(); // change to RETURN_MM_NULL()
}
```