False positive at Test* functions in expr-test.cc if a result is NULL

In be/src/exprs/expr-test.cc, some helper functions does not work unexpectedly.

If expr returns NULL, the test should fail because NULL cannot be compared. If we expect NULL as a result, we should use TestIsNull.
TestValue(expr, expr_type, expected_result)

TestValue("cast(null as int)", TYPE_INT, 0);
TestStringValue("cast(null as string)", "NULL");

Please don't forget to add the above cases as unit tests to verify your patch.