[fix] Return empty array instead of nil from QueryService.getServices

[sujalshah-bit]

Which problem is this PR solving?

Fixes #7921.

Description of the changes

When the backend has no traces, the getServices endpoint returned a nil services field, which is serialized as null in JSON.
This caused the UI to fail validation, since it expects an array.
This change ensures the services field is always returned as an empty array.

How was this change tested?

wrote a test TestQueryServiceGetServicesReturnsEmptySlice in service_test.go.

Checklist

• I have read https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
• I have signed all commits
• I have added unit tests for the new functionality
• I have run lint and test steps successfully
  • for jaeger: make lint test
  • for jaeger-ui: npm run lint and npm run test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.56%. Comparing base (435a23d) to head (0532162).
⚠️ Report is 18 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7925   +/-   ##
=======================================
  Coverage   95.56%   95.56%           
=======================================
  Files         316      316           
  Lines       16715    16718    +3     
=======================================
+ Hits        15973    15976    +3     
  Misses        579      579           
  Partials      163      163           

Flag	Coverage Δ
badger_v1	9.11% <ø> (ø)
badger_v2	1.90% <ø> (ø)
cassandra-4.x-v1-manual	13.41% <ø> (ø)
cassandra-4.x-v2-auto	1.89% <ø> (ø)
cassandra-4.x-v2-manual	1.89% <ø> (ø)
cassandra-5.x-v1-manual	13.41% <ø> (ø)
cassandra-5.x-v2-auto	1.89% <ø> (ø)
cassandra-5.x-v2-manual	1.89% <ø> (ø)
clickhouse	1.98% <ø> (ø)
elasticsearch-6.x-v1	17.27% <ø> (ø)
elasticsearch-7.x-v1	17.30% <ø> (ø)
elasticsearch-8.x-v1	17.45% <ø> (ø)
elasticsearch-8.x-v2	1.90% <ø> (ø)
elasticsearch-9.x-v2	1.90% <ø> (ø)
grpc_v1	8.44% <ø> (ø)
grpc_v2	1.90% <ø> (ø)
kafka-3.x-v2	1.90% <ø> (ø)
memory_v2	1.90% <ø> (ø)
opensearch-1.x-v1	17.34% <ø> (ø)
opensearch-2.x-v1	17.34% <ø> (ø)
opensearch-2.x-v2	1.90% <ø> (ø)
opensearch-3.x-v2	1.90% <ø> (ø)
query	1.90% <ø> (ø)
tailsampling-processor	0.54% <ø> (ø)
unittests	94.24% <100.00%> (+<0.01%)	⬆️

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[github-actions]

Metrics Comparison Summary

Total changes across all snapshots: 0

Detailed changes per snapshot

summary_metrics_snapshot_cassandra

📊 Metrics Diff Summary

Total Changes: 0

• 🆕 Added: 0 metrics
• ❌ Removed: 0 metrics
• 🔄 Modified: 0 metrics
• 🚫 Excluded: 106 metrics

summary_metrics_snapshot_cassandra

📊 Metrics Diff Summary

Total Changes: 0

• 🆕 Added: 0 metrics
• ❌ Removed: 0 metrics
• 🔄 Modified: 0 metrics
• 🚫 Excluded: 106 metrics

summary_metrics_snapshot_cassandra

📊 Metrics Diff Summary

Total Changes: 0

• 🆕 Added: 0 metrics
• ❌ Removed: 0 metrics
• 🔄 Modified: 0 metrics
• 🚫 Excluded: 53 metrics

summary_metrics_snapshot_cassandra

📊 Metrics Diff Summary

Total Changes: 0

• 🆕 Added: 0 metrics
• ❌ Removed: 0 metrics
• 🔄 Modified: 0 metrics
• 🚫 Excluded: 106 metrics

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[sujalshah-bit]

@yurishkuro could you kindly review :)

[Copilot]

Pull request overview

This PR fixes a bug where the getServices endpoint returns null instead of an empty array when the backend has no traces, which caused validation errors in the UI.

Changes:

• Modified GetServices method in QueryService to ensure it always returns an empty slice instead of nil
• Added a test case to verify that GetServices returns an empty slice when the backend returns nil

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
cmd/jaeger/internal/extension/jaegerquery/querysvc/service.go	Added nil-safety check to ensure GetServices always returns an empty slice instead of nil
cmd/jaeger/internal/extension/jaegerquery/querysvc/service_test.go	Added test case to verify GetServices returns empty slice when backend returns nil

Comments suppressed due to low confidence (1)

cmd/jaeger/internal/extension/jaegerquery/querysvc/service.go:134

• For consistency with GetServices, consider adding similar nil-safety handling to GetOperations. The v1adapter's GetOperations (at internal/storage/v2/v1adapter/tracereader.go:71) already checks for nil returns, suggesting that some storage backends may return nil for empty results. While not critical for this PR, adding the same defensive check here would prevent similar issues in the future and maintain consistency across the codebase.

func (qs QueryService) GetOperations(
	ctx context.Context,
	query tracestore.OperationQueryParams,
) ([]tracestore.Operation, error) {
	return qs.traceReader.GetOperations(ctx, query)
}

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