Fix another unbound variable error with set -u

This should have been part of the fix for pyenv#423, but I missed it because
I had PYENV_VIRTUALENV_DISABLE_PROMPT=1 set when doing my own testing.

Author: ackalker

bin/pyenv-sh-activate

@@ -229,8 +229,8 @@ if [ -z "${PYENV_VIRTUALENV_DISABLE_PROMPT}" ]; then
       echo "pyenv-virtualenv: prompt changing will be removed from future release. configure \`export PYENV_VIRTUALENV_DISABLE_PROMPT=1' to simulate the behavior." 1>&2
     fi
     cat <<EOS
-export _OLD_VIRTUAL_PS1="\${PS1}";
-export PS1="(${venv}) \${PS1}";
+export _OLD_VIRTUAL_PS1="\${PS1:-}";
+export PS1="(${venv}) \${PS1:-}";
 EOS
     ;;
   esac