Update reflectx to allow for optional nested structs

[ntbosscher]

Nested structs are now only instantiated when one of the database columns in that nested struct is not nil. This allows objects scanned in left/outer joins to keep their natural types (instead of setting everything to NullableX).

Example:

select house.id, owner.*,
from house
left join owner on owner.id = house.owner

Before

type House struct {
 ID      int
 Owner   *Person // if left join gives nulls, Owner is set
}

type Owner struct {
 ID sql.NullInt64 // make nullable columns even tho if table doesn't have those columns nullable
}

After

type House struct {
 ID      int
 Owner   *Person // if left join gives nulls, Owner will be nil
}

type Owner struct {
 ID int    // no need to set this to sql.NullInt
}