Collection.by_key should return DocumentNotFound for nil

Currently this results in a bad request when talking to ArangoDB:

```ruby
UsersCollection.by_key(123)
# #<Ashikawa::Core::DocumentNotFoundException: You requested a document from the server that does not exist>
>> UsersCollection.by_key(nil)
# #<Ashikawa::Core::BadSyntax: Status 400: The syntax of the request was bad>
```

For `nil` there should be a `DocumentNotFoundException` too.