Schema Select does not properly handle nested nullable fields

A select of a nested field should be nullable if any of its parents are nullable. So for example, a select of "a.b" should return a field named b that is nullable if either of a or b is nullable. Today we only examine b to see if the selected fields should be nullable.

Also the Select transform itself does not properly check for null values, and throws NullPointerExceptions when some row values are null.