Fix incorrect null_count in generate_unions_case integration test

[viirya]

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Part of #1404. This is last issue to enable generate_unions_case integration test case.

Currently running master branch with the test, getting an error:

Error message: Invalid: Arrow record batch 1 did not validate:
Invalid: In column 3: Invalid: Union child array #2 invalid: Invalid: null_count value (0) doesn't match actual number of nulls in array (2)

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