Make work in nounset (-u) mode

libexec/pyenv-sh-shell

@@ -49,7 +49,7 @@ if [ "$versions" = "--unset" ]; then
     echo "set -e PYENV_VERSION"
     ;;
   * )
-    echo 'PYENV_VERSION_OLD="$PYENV_VERSION"'
+    echo 'PYENV_VERSION_OLD="${PYENV_VERSION-}"'
     echo "unset PYENV_VERSION"
     ;;
   esac
@@ -110,7 +110,7 @@ if pyenv-prefix "${versions[@]}" >/dev/null; then
       echo "set -gx PYENV_VERSION \"$version\""
       ;;
     * )
-      echo 'PYENV_VERSION_OLD="$PYENV_VERSION"'
+      echo 'PYENV_VERSION_OLD="${PYENV_VERSION-}"'
       echo "export PYENV_VERSION=\"${version}\""
       ;;
     esac

test/shell.bats

@@ -47,7 +47,7 @@ load test_helper
   PYENV_SHELL=bash run pyenv-sh-shell --unset
   assert_success
   assert_output <<OUT
-PYENV_VERSION_OLD="\$PYENV_VERSION"
+PYENV_VERSION_OLD="\${PYENV_VERSION-}"
 unset PYENV_VERSION
 OUT
 }
@@ -75,7 +75,7 @@ SH
   PYENV_SHELL=bash run pyenv-sh-shell 1.2.3
   assert_success
   assert_output <<OUT
-PYENV_VERSION_OLD="\$PYENV_VERSION"
+PYENV_VERSION_OLD="\${PYENV_VERSION-}"
 export PYENV_VERSION="1.2.3"
 OUT
 }