LDEV-3640 QoQ needs to preserve nulls internally regardless of full null support

[bdw429s]

https://luceeserver.atlassian.net/browse/LDEV-3640

[bdw429s]

I made a few subtle, but important changes here

• stop detecting null support and returning strings internally. Just use nulls exclusively inside the query object-- they'll be converted later when the query is used if necessary
• Make sure I pass in a default value of null any time I access the value of a cell to prevent the null-default of an empty string kicking in
• Ensure all cell access is via QueryColumn.get(), which is the only way to get a a raw cell value and NOT have the nulls changed to empty strings.