add null value edge case to unit test for Row.equals

[jon-esperanza]

Addresses #19561
After #24773, it was discovered that there needs to be an improvement to the unit tests for Row.equals to test edge cases on null values

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[github-actions]

Checks are failing. Will not request review until checks are succeeding. If you'd like to override that behavior, comment assign set of reviewers

[Abacn]

Thank you!

[jon-esperanza]

@Abacn
beam_PreCommit_Java_Examples_Dataflow check was failing, is there a reason why that might be?