Fix unbound variable errors when running `pyenv activate` with `set -u` (#423)

bin/pyenv-sh-deactivate

@@ -134,7 +134,7 @@ EOS
   ;;
 * )
   cat <<EOS
-if [ -n "\${_OLD_VIRTUAL_PATH}" ]; then
+if [ -n "\${_OLD_VIRTUAL_PATH:-}" ]; then
   export PATH="\${_OLD_VIRTUAL_PATH}";
   unset _OLD_VIRTUAL_PATH;
 fi;
@@ -153,7 +153,7 @@ EOS
   ;;
 * )
   cat <<EOS
-if [ -n "\${_OLD_VIRTUAL_PYTHONHOME}" ]; then
+if [ -n "\${_OLD_VIRTUAL_PYTHONHOME:-}" ]; then
   export PYTHONHOME="\${_OLD_VIRTUAL_PYTHONHOME}";
   unset _OLD_VIRTUAL_PYTHONHOME;
 fi;
@@ -167,7 +167,7 @@ fish )
   ;;
 * )
   cat <<EOS
-if [ -n "\${_OLD_VIRTUAL_PS1}" ]; then
+if [ -n "\${_OLD_VIRTUAL_PS1:-}" ]; then
   export PS1="\${_OLD_VIRTUAL_PS1}";
   unset _OLD_VIRTUAL_PS1;
 fi;