Fix NPE when fetching a non-exist key from a GroupBy w/ derivation

[xiaohui-sun]

Summary

There is a NPE when fetching a non-exist key from a GroupBy w/ derivation like below:

2025-02-11 18:07:04 ERROR Fetcher:192 - Exception Message: scala.MatchError: (Map(id_host -> 12345),null) (of class scala.Tuple2)
	at ai.chronon.online.OnlineDerivationUtil$.$anonfun$buildRenameOnlyDerivationFunction$1(OnlineDerivationUtil.scala:32)
	at ai.chronon.online.FetcherBase.$anonfun$fetchGroupBys$25(FetcherBase.scala:514)
	at scala.util.Try$.apply(Try.scala:210)
	at ai.chronon.online.FetcherBase.$anonfun$fetchGroupBys$18(FetcherBase.scala:516)
	at scala.util.Success.map(Try.scala:262)
	at ai.chronon.online.FetcherBase.$anonfun$fetchGroupBys$17(FetcherBase.scala:449)
	at scala.collection.Iterator$$anon$9.next(Iterator.scala:575)
	at scala.collection.immutable.List.prependedAll(List.scala:153)
	at scala.collection.IterableOnceOps.toList(IterableOnce.scala:1251)
	at scala.collection.IterableOnceOps.toList$(IterableOnce.scala:1251)
	at scala.collection.AbstractIterator.toList(Iterator.scala:1288)
	at ai.chronon.online.FetcherBase.$anonfun$fetchGroupBys$9(FetcherBase.scala:447)
	at scala.concurrent.impl.Promise$Transformation.run(Promise.scala:431)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:750)

This PR fixes it by returning null values for derivations if key is not found.

Why / Goal

Test Plan

• Added Unit Tests
• Covered by existing CI
• Integration tested

Checklist

• Documentation update

Reviewers

@hzding621 @yuli-han