fix error when submission is empty
fix error when submission is empty

  This will check for an empty submission before attempting to pull the
  upload file attributes out. Note that we should still disable the
  submission button until a valid submission has been created (a link,
  text, or a uploaded file must be present.)

mkoon/gradecraft-development,UM-USElab/gradecraft-development,UM-USElab/gradecraft-development,UM-USElab/gradecraft-development,mkoon/gradecraft-development,UM-USElab/gradecraft-development,mkoon/gradecraft-development