Fix bug in neutral PFO thinning

!37239 (merged) (October 26, 2020) introduced a bug to ThinNegativeEnergyNeutralPFOsAlg which incorrectly disabled thinning of PFOs if no FlowElement container was provided. This fixes the issue, such that the FlowElement and PFO thinning are correctly independent of each other.

Tagging @jferrand, @miochoa, and @mhodgkin as this fixes the neutral PFO container size increase seen in the November validation.

PhysicsAnalysis/AnalysisCommon/ThinningUtils/src/ThinNegativeEnergyNeutralPFOsAlg.cxx

@@ -102,7 +102,7 @@ StatusCode ThinNegativeEnergyNeutralPFOsAlg::execute()
 
     const EventContext& ctx = Gaudi::Hive::currentContext();
     
-    if(!m_neutralPFOsFEKey.key().empty()){
+    if(!m_neutralPFOsKey.key().empty()){
 
         // Retrieve the container
         SG::ThinningHandle<xAOD::PFOContainer> neutralPFOs (m_neutralPFOsKey, ctx);