Failed to search 0 value

Hi,
I was using this repository to find a boolean value which is stored as integer in MySQL thorugh RequestCriteria. But it failed.
After digging deeper to the source code, I found that the code checks if search or not by its content:
```php
...
if ($search && is_array($fieldsSearchable) && count($fieldsSearchable)) {
}
...
```
[code](
https://github.com/andersao/l5-repository/blob/f2008d8ff8af50036d2d155c0b5dbb07cbcd0ee4/src/Prettus/Repository/Criteria/RequestCriteria.php#L49)

So if you are searching a value which is [falsy](http://php.net/manual/en/language.types.boolean.php#language.types.boolean.casting), then it won't search.

The solution is simple, use `is_null` to check the `$search`:
```php
if (!is_null($search) && is_array($fieldsSearchable) && count($fieldsSearchable)) {
}
```

Thanks.