JoinNullable deserialisation fix

[NPCRUS]

In the current implementation, JoinNullable in order to determine that R shouldBe Option or Some is deserialising whole set and then counting nulls. In cases where you have user written TypeMapper, that doesn't do nulls tricks(like inside Dialects.scala), such:

enum Enumeration {
 case A, B, C
}
object Enumeration {
 given TypeMapper[Enumeration] = StringType.bimap(_.toString(), Enumeration.valueOf)
}

then leftJoin query, where the right side can be nullable instead of returning (A, None) will fail with exception, check new test in DataTypesTests

new implementation will check first if the rest of columns are null without deserialising them into type and only if one of them is not - will try to construct

comments on everything are welcome!

fixes #65

[lihaoyi]

Looks reasonable to me, just run the autofix commands then we can merge this I think

[NPCRUS]

@lihaoyi ping

[lihaoyi]

Thanks! Feel free to ping me when you're done with your PRs and I can cut a stable version for you to depend on