fix export for openassessment drafts (PLAT-249)

This addresses an issue with exporting the drafts of xblocks that don't have `xml_attributes` defined for them

@dmitchell 
@nasthagiri 

https://openedx.atlassian.net/browse/PLAT-249
@adampalay Can you confirm that the new unit test executes this new line?  It appears that it's not being run in the coverage report.