[SPARK-4493][SQL] Tests for IsNull / IsNotNull in the ParquetFilterSuite

This is a follow-up of #3367 and #3644.

At the time #3644 was written, #3367 hadn't been merged yet, thus `IsNull` and `IsNotNull` filters are not covered in the first version of `ParquetFilterSuite`. This PR adds corresponding test cases.

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Author: Cheng Lian <lian@databricks.com>

Closes #3748 from liancheng/test-null-filters and squashes the following commits:

1ab943f [Cheng Lian] IsNull and IsNotNull Parquet filter test case for boolean type
bcd616b [Cheng Lian] Adds Parquet filter pushedown tests for IsNull and IsNotNull