Fix FPE in AmgVector eta() implementation (ATLASRECTS-7646)

John Derek Chapmanrequested to merge
jchapman/athena:FixATLASRECTS-7646_23.0 into 23.0

This time the FPE is caused by the case that the rho component (=x*x + y*y) is negligible compared to the z component, such that m-z = 0.0 to double-precision (where m = sqrt(rho*rho + z*z)).

Taggin @christos

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