FlowETL DataPresentSensor trips over null fields

[jc-harrison]

The FlowETL DataPresentSensor operator checks whether a file contains any data by attempting to select one row from the foreign data wrapper around the file. DataPresentSensor is an Airflow SqlSensor operator, whose success criterion is that the first cell in the returned row is not in (0, '0', '', None). This means that if the first field in the first row of the file is null (or an empty string), the DataPresentSensor operator will fail to detect data, so ingestion will not proceed.

Given that it's not necessarily the case that a null value in the first column (whatever that may be) invalidates the entire contents of the file, we should find a more robust way of handling this check.

[jc-harrison]

See also #5763