[SPARK-8759][SQL] add default eval to binary and unary expression according to default behavior of nullable

We have `nullSafeCodeGen` to provide default code generation for binary and unary expression, and we can do the same thing for `eval`.

Author: Wenchen Fan <cloud0fan@outlook.com>

Closes #7157 from cloud-fan/refactor and squashes the following commits:

f3987c6 [Wenchen Fan] refactor Expression