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

[liancheng]

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.

[SparkQA]

Test build #24662 has started for PR 3748 at commit 1ab943f.

• This patch merges cleanly.

[SparkQA]

Test build #24662 has finished for PR 3748 at commit 1ab943f.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
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https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/24662/
Test PASSed.

[liancheng]

@marmbrus This is ready to go.