CSW Harvest from Geoserver requires valid dateStamp

When harvesting from Geoserver csw it seems as if a valid date in `dateStamp.DateTime` is required for the harvest to work. This is an issue as Geoserver only sets `dateStamp.Date` and has it default to "Unknown" if `metadata.date` is null.
This was very hard to debug as there seems to be no error message addressing this issue.

Start by reproducing a CSW harvest against GeoServer metadata where dateStamp contains only a date or "Unknown". Trace the dateStamp validation and error handling to determine the expected behavior; done should include either successful harvesting of this response or a clear, actionable error.

Written by the indexing model from the issue text.