MagFieldInterfaces: Suppress cppcheck warning.

Suppress cppcheck false positive.

MagneticField/MagFieldInterfaces/MagFieldInterfaces/IMagFieldSvc.h

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2023 CERN for the benefit of the ATLAS collaboration
 */
 
 ///////////////////////////////////////////////////////////////////
@@ -46,6 +46,7 @@ namespace MagField {
           getField( xyz->data(), bxyz->data(), nullptr );
         }
         void getField(const Amg::Vector3D *xyz, Amg::Vector3D *bxyz, Amg::RotationMatrix3D *deriv) const {
+          // cppcheck-suppress nullPointer; false positive
           getField( xyz->data(), bxyz->data(), deriv->data() );
         }