Do not remove special directories from the search path even if empty
LbEnv.sh
is based on https://gitlab.cern.ch/gaudi/xenv, which cleans up useless entries from *PATH variables.
While it's very useful for the runtime environment use case, it may be very inconvenient for LbEnv when bootstrapping an installation, because the generated environment will be empty until we actually install something, at which point we will have to logout and login again to get a usable environment.