Merge branch 'cherry-pick-ba82e91f-22.0' into '22.0'

Sweeping !56833 from master to 22.0.
iPatFitter - Check if the parameters actually exist before returning themCloses ATLASRECTS-7280

See merge request atlas/athena!56884

Tracking/TrkFitter/TrkiPatFitterUtils/TrkiPatFitterUtils/FitParameters.h

@@ -213,7 +213,8 @@ FitParameters::d0(void) const
 inline double
 FitParameters::difference(int param) const
 {
-  // if (! m_differences)  return 0.;   // surely unnecessary?
+   if (! m_differences)  return 0.;
+   /// Use as_const to avoid compiler warning
   return std::as_const(*m_differences)(param);
 }