Initialize NullValueHandlingConfig for failed tests

[a2l007]

When run independently, tests DelegateOrMinKeyCollectorTest and QuantilesSketchKeyCollectorTest currently fail with the following error:

[ERROR] Errors: 
[ERROR] org.apache.druid.msq.statistics.DelegateOrMinKeyCollectorTest.testAdd
[ERROR]   Run 1: DelegateOrMinKeyCollectorTest.testAdd:81->createKey:148 » IllegalState NullHandling module not initialized, call NullHandling.initializeForTests()
[ERROR] org.apache.druid.msq.statistics.DelegateOrMinKeyCollectorTest.testDownSampleSingleKey
[ERROR]   Run 1: DelegateOrMinKeyCollectorTest.testDownSampleSingleKey:99->createKey:148 » IllegalState NullHandling module not initialized, call NullHandling.initializeForTests()
[ERROR] org.apache.druid.msq.statistics.DelegateOrMinKeyCollectorTest.testDownSampleTwoKeys
[ERROR]   Run 1: DelegateOrMinKeyCollectorTest.testDownSampleTwoKeys:125->createKey:148 » IllegalState NullHandling module not initialized, call NullHandling.initializeForTests()
[ERROR] org.apache.druid.msq.statistics.QuantilesSketchKeyCollectorTest.test_sequentialKeys_unweighted
[ERROR]   Run 1: QuantilesSketchKeyCollectorTest.test_sequentialKeys_unweighted:66 » IllegalState NullHandling module not initialized, call NullHandling.initializeForTests()
[ERROR] org.apache.druid.msq.statistics.QuantilesSketchKeyCollectorTest.test_uniformRandomKeys_barbellWeighted
[ERROR]   Run 1: QuantilesSketchKeyCollectorTest.test_uniformRandomKeys_barbellWeighted:136 » IllegalState NullHandling module not initialized, call NullHandling.initializeForTests()
[ERROR] org.apache.druid.msq.statistics.QuantilesSketchKeyCollectorTest.test_uniformRandomKeys_inverseBarbellWeighted
[ERROR]   Run 1: QuantilesSketchKeyCollectorTest.test_uniformRandomKeys_inverseBarbellWeighted:163 » IllegalState NullHandling module not initialized, call NullHandling.initializeForTests()
[ERROR]   Run 1: QuantilesSketchKeyCollectorTest.test_uniformRandomKeys_unweighted:85 » IllegalState NullHandling module not initialized, call NullHandling.initializeForTests()
ERROR] org.apache.druid.msq.statistics.QuantilesSketchKeyCollectorTest.test_uniformRandomKeys_unweighted_downSampledToSmallestSize
[ERROR]   Run 1: QuantilesSketchKeyCollectorTest.test_uniformRandomKeys_unweighted_downSampledToSmallestSize:103 » IllegalState NullHandling module not initialized, call NullHandling.initializeForTests()

This PR initializes the NullValueHandlingConfig to fix these tests.

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This PR has:

• been self-reviewed.

[kfaraz]

Following the example of other tests, I think it would be better to do the NullHandling.initializeForTests() in the test class themselves rather than the utils class.

[a2l007]

@kfaraz Thanks for the review. Refactored the tests based on your comment.

[kfaraz]

Thanks for the fix, @a2l007 🚀

[kfaraz]

@a2l007 , you might need to merge the latest master to fix the build failures using #13083