Add protection for photons with no layer 1 EM clusters for pointing tool

The photon pointing tool might run over photons which don't have any EM1 cluster. This would trigger a warning from the ShowerDepthTool. https://gitlab.cern.ch/atlas/athena/-/blob/main/Reconstruction/egamma/egammaUtils/Root/ShowerDepthTool.cxx#L111

To avoid this an early check is added here, returning the same value as the ShowerDepthTool would but avoiding the warning.

FYI @christos @atishelm @gdigrego