[SPARK-7527] [CORE] Fix createNullValue to return the correct null values and REPL mode detection

The root cause of SPARK-7527 is `createNullValue` returns an incompatible value `Byte(0)` for `char` and `boolean`.

This PR fixes it and corrects the class name of the main class, and also adds an unit test to demonstrate it.

Author: zsxwing <zsxwing@gmail.com>

Closes #6735 from zsxwing/SPARK-7527 and squashes the following commits:

bbdb271 [zsxwing] Use pattern match in createNullValue
b0a0e7e [zsxwing] Remove the noisy in the test output
903e269 [zsxwing] Remove the code for Utils.isInInterpreter == false
5f92dc1 [zsxwing] Fix createNullValue to return the correct null values and REPL mode detection