[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){

	zval *option, *options, *value;

	PHALCON_MM_GROW();

	phalcon_fetch_params(1, 1, 0, &option);
	
	PHALCON_OBS_VAR(options);
	phalcon_read_property_this(&options, this_ptr, SL("_options"), PH_NOISY_CC);
	if (phalcon_array_isset(options, option)) {
		PHALCON_OBS_VAR(value);
		phalcon_array_fetch(&value, options, option, PH_NOISY);
		RETURN_CCTOR(value);
	}
	
	RETURN_MM_EMPTY_STRING();
}
```