Don't require IsZombie on the open BDT file in egammaMVACalib
egammaMVACalib requires IsZombie() on an open BDT file for it to be valid, but this would imply the tool only correctly initializes if the file is corrupted? This seems wrong, so I've made a one-liner fix. N.B.: this issue also seems to exist in 21.2 for the tool of the same name, so it's not clear why this tool has ever worked