ActsGeometry: fix cppcheck uninitialized variables

Tracking/Acts/ActsGeometry/ActsGeometry/ActsMaterialTrackWriterSvc.h

 /*
-  Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2024 CERN for the benefit of the ATLAS collaboration
 */
 
 #ifndef ACTSGEOMETRY_ACTSMATERIALTRACKWRITERSVC_H
@@ -38,19 +38,19 @@ private:
   std::mutex m_writeMutex;
   std::thread m_writeThread;
   std::atomic<bool> m_doEnd;
-  TFile* p_tFile;
-  TTree* p_tree;
-
-  float m_v_x;    ///< start global x
-  float m_v_y;    ///< start global y
-  float m_v_z;    ///< start global z
-  float m_v_px;   ///< start global momentum x
-  float m_v_py;   ///< start global momentum y
-  float m_v_pz;   ///< start global momentum z
-  float m_v_phi;  ///< start phi direction
-  float m_v_eta;  ///< start eta direction
-  float m_tX0;    ///< thickness in X0/L0
-  float m_tL0;    ///< thickness in X0/L0
+  TFile* p_tFile{};
+  TTree* p_tree{};
+
+  float m_v_x{};    ///< start global x
+  float m_v_y{};    ///< start global y
+  float m_v_z{};    ///< start global z
+  float m_v_px{};   ///< start global momentum x
+  float m_v_py{};   ///< start global momentum y
+  float m_v_pz{};   ///< start global momentum z
+  float m_v_phi{};  ///< start phi direction
+  float m_v_eta{};  ///< start eta direction
+  float m_tX0{};    ///< thickness in X0/L0
+  float m_tL0{};    ///< thickness in X0/L0
 
   std::vector<float> m_step_sx;      ///< step x (start) position (optional)
   std::vector<float> m_step_sy;      ///< step y (start) position (optional)

Tracking/Acts/ActsGeometry/ActsGeometry/ActsPropStepRootWriterSvc.h

 /*
-  Copyright (C) 2002-2018 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2024 CERN for the benefit of the ATLAS collaboration
 */
 
 #ifndef ACTSGEOMETRY_ACTSPROPSTEPROOTWRITERSVC_H
@@ -69,9 +69,9 @@ private:
   //Gaudi::Property<bool> m_writePassive{this, "WritePassive", true, ""};
 
   // root branch storage
-    TFile*             m_outputFile;    ///< the output file
-    TTree*             m_outputTree;    ///< the output tree
-    int m_eventNum;
+    TFile*             m_outputFile{};    ///< the output file
+    TTree*             m_outputTree{};    ///< the output tree
+    int m_eventNum{};
     std::vector<float> m_s_pX;   ///< global position x of the step
     std::vector<float> m_s_pY;   ///< global position y of the step
     std::vector<float> m_s_pZ;   ///< global position z of the step