XML grade exporting display ignores activity ID number

In order to export grades using XML format users and activities must have an ID number. However, if you don't select any activity, it will export grades because it is validating only if the user ID number is not empty.

It was discovered while testing MDLQA-7306.

1. Create a course, an activity and enrol few users.
2. Edit these users and the activity and give then an ID number
3. Give these users some grades.
4. Go to Grades > Export > XML file and unselect all activities checkboxes and click Download.

Expected behaviour

You should not be able to see any grade, since you didn't select any activity.

Present behaviour

The report ignores the checkboxes and exports few grades.

I also notice the report exports grades with closing tag without the opening tag. (see assignment)

<result>

    <state>unknown</state>

    <assignment/>

    <student>667</student>

    <score>-</score>

</result>