Incorrect Not predicate evaluation in ORC filtering pushdown

**Describe the bug**
The `Not` predicate evaluation in row group filtering was incorrect.
https://github.com/datafusion-contrib/orc-rust/pull/78

**To Reproduce**
```scala
test("test string type filter for orc table") {
withTable("orc_string_filter") {
sql("create table orc_string_filter(id int, b string) using orc")
sql("insert into orc_string_filter values (1, 'abc'), (2, null), (3, 'def')")
checkSparkAnswerAndOperator("select * from orc_string_filter where b is not null")
}
}
```
**Additional context**
Perhaps we should temporarily disable ORC predicate pushdown.