Bash test difference (-z/-n)

test.sh

check() {
  # -z: True when the string length is 0
  if [ -z $DUMMY ]; then
    echo '-z $DUMMY'
  fi
  if [ -z "$DUMMY" ]; then
    echo '-z "$DUMMY"'
  fi
  if [ -z "${DUMMY+xxx}" ]; then
    echo '-z "${DUMMY+xxx}"'
  fi
  if [ -z "${DUMMY:+xxx}" ]; then
    echo '-z "${DUMMY:+xxx}"'
  fi

  # -n: True when the string length is greater than 0
  if [ -n $DUMMY ]; then
    echo '-n $DUMMY'
  fi
  if [ -n "$DUMMY" ]; then
    echo '-n "$DUMMY"'
  fi
  if [ -n "${DUMMY+xxx}" ]; then
    echo '-n "${DUMMY+xxx}"'
  fi
  if [ -n "${DUMMY:+xxx}" ]; then
    echo '-n "${DUMMY:+xxx}"'
  fi
}

echo
echo '# When $DUMMY is not defined'
unset DUMMY
check

echo
echo '# When $DUMMY is an empty string'
export DUMMY=""
check

echo
echo '# When $DUMMY is a non-empty string'
export DUMMY="foo"
check

# When $DUMMY is not defined
-z $DUMMY
-z "$DUMMY"
-z "${DUMMY+xxx}"
-z "${DUMMY:+xxx}"
-n $DUMMY

# When $DUMMY is an empty string
-z $DUMMY
-z "$DUMMY"
-z "${DUMMY:+xxx}"
-n $DUMMY
-n "${DUMMY+xxx}"

# When $DUMMY is a non-empty string
-n $DUMMY
-n "$DUMMY"
-n "${DUMMY+xxx}"
-n "${DUMMY:+xxx}"