Protection against DIVBYZERO FPEs in AmgMatrixPlugin.h eta() method

Previously this function had no protection, now protection has been added following the conventions in ROOT 6.26.6:

• https://github.com/root-project/root/blob/v6-26-06/math/genvector/inc/Math/GenVector/eta.h
• https://github.com/root-project/root/blob/v6-26-06/math/genvector/inc/Math/GenVector/etaMax.h

It should be noted that CLHEP uses a different convention: https://gitlab.cern.ch/CLHEP/CLHEP/-/blob/develop/Vector/src/ThreeVector.cc#L58-64

This should fix ATLASSIM-6423.

The change does not seem to alter the output of the CI tests, so marking it as ready.

Thanks to @christos for the discussions.