when the robot receives a folder share, ensure there is at least one .xml file present before moving to incoming

[mengwong]

sometimes people run through the tutorial and produce a folder that contains only a README file, because some error (usually lack of data entry) prevents any XML files from being generated.

When the robot receives a sharing assignment for such a folder, before moving it into the incoming folder, first check to make sure there are XML files in the folder.

the relevant script is
https://github.com/legalese/legalese-google-app/blob/master/robot-legalese.gs

it is attached to the browser spreadsheet.