Fix GetServices never returns nil

Some storage backends return nil when no services exist, which caused
the getServices API to omit the services field or serialize it as null.
This breaks the UI schema expectations, which always require an array.

Normalize nil to an empty slice in QueryService.GetServices so that
all callers receive a consistent, non-nil response.

Fixes #7921.

Signed-off-by: Sujal Shah <sujalshah28092004@gmail.com>

Author: sujalshah-bit

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

@@ -119,7 +119,11 @@ func (qs QueryService) GetTraces(
 }
 
 func (qs QueryService) GetServices(ctx context.Context) ([]string, error) {
-	return qs.traceReader.GetServices(ctx)
+	services, err := qs.traceReader.GetServices(ctx)
+	if services == nil {
+		services = []string{}
+	}
+	return services, err
 }
 
 func (qs QueryService) GetOperations(

cmd/jaeger/internal/extension/jaegerquery/querysvc/service_test.go

@@ -749,3 +749,17 @@ func TestGetCapabilities(t *testing.T) {
 		})
 	}
 }
+func TestQueryServiceGetServicesReturnsEmptySlice(t *testing.T) {
+	reader := new(tracestoremocks.Reader)
+	reader.
+		On("GetServices", mock.Anything).
+		Return(nil, nil).Once()
+
+	qs := NewQueryService(reader, nil, QueryServiceOptions{})
+
+	services, err := qs.GetServices(context.Background())
+
+	require.NoError(t, err)
+	require.NotNil(t, services)
+	require.Equal(t, 0, len(services))
+}