doesntHave returns incorrect data on to-one relationships when foreign key is null

[jclausen]

doesntHave currently creates a WHERE NOT EXISTS sub-select on a to-one relationship that looks like this:

SELECT * 
FROM [myTable] 
WHERE NOT EXISTS (SELECT 1 FROM [myOtherTable] WHERE ([myOtherTable].[id] = [FK_otherTable]))

This means that if FK_otherTable is null, it will return all of those records, in addition to any where the empty does not exist.

The workaround, of course, is to use whereNull( 'FK_otherTable' ) but it seems like doesntHave should be able to handle belongsTo relationships