Fixing TimeSeriesUnionQueryRunnerTest by extending InitializedNullHandlingTest

[harinirajendran]

Fixing the following unit test failure

[ERROR] Tests run: 1, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 0.01 s <<< FAILURE! - in org.apache.druid.query.timeseries.TimeSeriesUnionQueryRunnerTest00:44
[ERROR] initializationError(org.apache.druid.query.timeseries.TimeSeriesUnionQueryRunnerTest)  Time elapsed: 0.01 s  <<< ERROR!00:44
java.lang.IllegalStateException: NullHandling module not initialized, call NullHandling.initializeForTests()00:44
	at org.apache.druid.common.config.NullHandling.replaceWithDefault(NullHandling.java:71)00:44
	at org.apache.druid.segment.column.ColumnCapabilitiesImpl.createSimpleNumericColumnCapabilities(ColumnCapabilitiesImpl.java:165)00:44
	at org.apache.druid.segment.incremental.IncrementalIndex$MetricDesc.<init>(IncrementalIndex.java:1102)00:44
	at org.apache.druid.segment.incremental.IncrementalIndex.<init>(IncrementalIndex.java:283)```