Merge pull request #814 from javabrett/safe-PYTHONPATH

Made variable reference PYTHONPATH safe in case of set -eu.

base-notebook/start.sh

@@ -102,7 +102,7 @@ if [ $(id -u) == 0 ] ; then
     # the environment preserved
     run-hooks /usr/local/bin/before-notebook.d
     echo "Executing the command: ${cmd[@]}"
-    exec sudo -E -H -u $NB_USER PATH=$PATH XDG_CACHE_HOME=/home/$NB_USER/.cache PYTHONPATH=$PYTHONPATH "${cmd[@]}"
+    exec sudo -E -H -u $NB_USER PATH=$PATH XDG_CACHE_HOME=/home/$NB_USER/.cache PYTHONPATH=${PYTHONPATH:-} "${cmd[@]}"
 else
     if [[ "$NB_UID" == "$(id -u jovyan)" && "$NB_GID" == "$(id -g jovyan)" ]]; then
         # User is not attempting to override user/group via environment