fix: Fix NPE while querying column having null value

[saxenakshitiz]

This change fixes NPE while retrieving column having null value. The exception trace is as follows -

	at org.hypertrace.core.query.service.api.Value$Builder.setString(Value.java:1598) ~[query-service-api.jar:?]
	at org.hypertrace.core.query.service.postgres.PostgresBasedRequestHandler.convert(PostgresBasedRequestHandler.java:471) ~[query-service-impl.jar:?]
	at org.hypertrace.core.query.service.postgres.PostgresBasedRequestHandler.handleRequest(PostgresBasedRequestHandler.java:390) ~[query-service-impl.jar:?]
	at org.hypertrace.core.query.service.QueryServiceImpl.lambda$executeTransformedRequest$4(QueryServiceImpl.java:100) ~[query-service-impl.jar:?]
	at io.reactivex.rxjava3.internal.operators.mixed.MaybeFlatMapObservable$FlatMapObserver.onSuccess(MaybeFlatMapObservable.java:102) ~[rxjava-3.1.5.jar:?]
	at io.reactivex.rxjava3.internal.operators.maybe.MaybeSwitchIfEmpty$SwitchIfEmptyMaybeObserver.onSuccess(MaybeSwitchIfEmpty.java:75) ~[rxjava-3.1.5.jar:?]
	at io.reactivex.rxjava3.internal.operators.maybe.MaybeJust.subscribeActual(MaybeJust.java:36) ~[rxjava-3.1.5.jar:?]
	at io.reactivex.rxjava3.core.Maybe.subscribe(Maybe.java:5375) ~[rxjava-3.1.5.jar:?]
	at io.reactivex.rxjava3.internal.operators.maybe.MaybeSwitchIfEmpty.subscribeActual(MaybeSwitchIfEmpty.java:38) ~[rxjava-3.1.5.jar:?]
	at io.reactivex.rxjava3.core.Maybe.subscribe(Maybe.java:5375) ~[rxjava-3.1.5.jar:?]
	at io.reactivex.rxjava3.internal.operators.mixed.MaybeFlatMapObservable.subscribeActual(MaybeFlatMapObservable.java:49) ~[rxjava-3.1.5.jar:?]
	at io.reactivex.rxjava3.core.Observable.subscribe(Observable.java:13173) ~[rxjava-3.1.5.jar:?]
	at io.reactivex.rxjava3.internal.operators.observable.ObservableLift.subscribeActual(ObservableLift.java:58) ~[rxjava-3.1.5.jar:?]
	at io.reactivex.rxjava3.core.Observable.subscribe(Observable.java:13173) ~[rxjava-3.1.5.jar:?]
	at io.reactivex.rxjava3.internal.operators.mixed.SingleFlatMapObservable$FlatMapObserver.onSuccess(SingleFlatMapObservable.java:110) ~[rxjava-3.1.5.jar:?]
	at io.reactivex.rxjava3.internal.operators.single.SingleFlatMap$SingleFlatMapCallback$FlatMapSingleObserver.onSuccess(SingleFlatMap.java:112) ~[rxjava-3.1.5.jar:?]
	at io.reactivex.rxjava3.internal.operators.single.SingleFlatMap$SingleFlatMapCallback$FlatMapSingleObserver.onSuccess(SingleFlatMap.java:112) ~[rxjava-3.1.5.jar:?]

[codecov]

Codecov Report

Merging #160 (76d86f8) into main (d9a094d) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main     #160      +/-   ##
============================================
+ Coverage     77.87%   77.91%   +0.03%     
  Complexity      758      758              
============================================
  Files            79       79              
  Lines          3300     3305       +5     
  Branches        368      369       +1     
============================================
+ Hits           2570     2575       +5     
  Misses          568      568              
  Partials        162      162              

Flag	Coverage Δ
integration	77.91% <100.00%> (+0.03%)	⬆️
unit	70.29% <33.33%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ertrace/core/query/service/QueryServiceConfig.java	92.64% <100.00%> (+0.22%)	⬆️
.../service/postgres/PostgresBasedRequestHandler.java	39.66% <100.00%> (+0.75%)	⬆️

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[kotharironak]

Note: The null string is set in QS o/p in other handlers, so this fix will align postgres impl.

[github-actions]

Unit Test Results

  38 files  ±0    38 suites  ±0   9s ⏱️ -2s
246 tests +1  246 ✔️ +1  0 💤 ±0  0 ❌ ±0 

Results for commit a8fbb97. ± Comparison against base commit d9a094d.