getAdditionalAttributeTable returning empty string

In the

/vendor/magento/module-eav/Model/ResourceModel/Attribute/Collection.php

file, the call below is simply returning an empty string, any ideas as to where this might be, or even where this function is implemented? I’ve tried using Grep etc to find where its implemented but that hasnt been able to find it.

$ extraTable = $ entityType->getAdditionalAttributeTable();