Downloading a report with no rows via a webservice should return an empty file instead of a error message.

[abias]

Prerequisites:

Do everything necessary to download reports via webservice (see https://github.com/moodleou/moodle-report_customsql/blob/main/internaldoc/functionality.txt#L71): You have to especially enable webservices, create a webservice user, generate a webservice token for this user, allow the webservice token to download files

Steps to reproduce:

• Create a manual report which accepts a query parameter. For example, use this query which gets all users who have completed a course since the given timestamp:

  SELECT cc.userid AS userid, 
     cc.course AS courseid, 
     cc.timecompleted AS completiontime
  FROM {course_completions} AS cc
  WHERE timecompleted IS NOT NULL 
  AND timecompleted >= :fromtimestamp

• Retrieve a report export via webservice which is expected to return at least one row:

  https://mymoodle.com/webservice/pluginfile.php/1/report_customsql/download/1?token=ze98rghkjdfhzehdgkjhfgkjdhg98ehgdnrglk&dataformat=csv&fromtimestamp=1653907942

• Retrieve a report export via webservice which is expected to return zero rows by changing the query parameter:

  https://mymoodle.com/webservice/pluginfile.php/1/report_customsql/download/1?token=ze98rghkjdfhzehdgkjhfgkjdhg98ehgdnrglk&dataformat=csv&fromtimestamp=1653908483

Expected result:

• The report export with rows should return a CSV file with the CSV header and all data rows.
• The report export without rows should return a CSV file without any data row.

Actual result:

• The report export with rows returns a CSV file with the CSV header and all data rows.
• The report export without rows return error message saying:

  {
  "error": "Unbekannte Download-Datei.",
  "errorcode": "unknowndownloadfile",
  "stacktrace": null,
  "debuginfo": null,
  "reproductionlink": null
  }

Analysis:

• In https://github.com/moodleou/moodle-report_customsql/blob/main/locallib.php#L120, the CSV header is not written until the first data row is retrieved from the database.
• As no CSV file is created, on https://github.com/moodleou/moodle-report_customsql/blob/master/lib.php#L98 an error message is returned to the webservice caller