Update mirror.sh to fix DELAY Override (#124)

This primarily fix situation where DELAY override is incorrect.

Previous behaviors:
* If DELAY is unset, will set DELAY_ARG to "--delay=" (empty string after equal sign) 
* If DELAY is set, DELAY_ARG will be set to "--delay=10000" if NVD_API_KEY unspecified and will not correctly set DELAY value

vulnz/src/docker/scripts/mirror.sh

@@ -7,7 +7,7 @@ if [ -z $NVD_API_KEY ]; then
   DELAY_ARG="--delay=10000"
 fi
 
-if [ -z $DELAY ]; then
+if [ -n "${DELAY}" ]; then
   DELAY_ARG="--delay=$DELAY"
 fi