Fix: attachment check covers null as well as undefined

[cahirodoherty-learningpool]

Fixes #https://github.com/adaptlearning/adapt-contrib-xapi/issues/115

Fix

• Check for attachment parameter now correctly handles a null value

Testing

1. Create a course with xapi extension and a custom plugin that will add an 'attachments' array of objects to the xapi statement. Attachment objects should be of the form outlined here - https://www.npmjs.com/package/xapiwrapper/v/1.11.0#send-statement-with-attachments

[github-actions[bot]]

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