Don't require empty DRIP_INIT just to use DRIP_INIT_CLASS

This commit defaults the value of DRIP_INIT to an empty string in the event the environment variable isn't set. This removes the stumbling block of having to set an empty DRIP_INIT environment variable just to get Drip to respect the DRIP_INIT_CLASS environment variable.

This could also be done in the drip shell script but this seemed like a better place to put it.