fix DRIP_INIT not being set when LEIN_JAVA_CMD is drip

I checked the drip code, and discovered you have to have DRIP_INIT_CLASS and DRIP_INIT both set, otherwise DRIP_INIT will get reset to the default of "-e nil". Adding this fix cuts the startup time in half for me when running "lein repl" with drip (~2s to ~1s). 

I also added a check to not overwrite DRIP_INIT if it was already set by the user beforehand.