The truth about -z and -n

if [ -z $null ]
then
echo '$null is null'
else
echo '$null file is not null.'
fi

if [ -n $null ]
then
echo '$null file is not null.'
else
echo '$null is null'
fi

The output is
$$null is null
$$null file is not null.

Not quite as we expected,So we learn that we should always quote the tested string and -n needs it.

if [ -z $null ]
then
echo '$null is null'
else
echo '$null file is not null.'
fi

if [ -n "$null" ]
then
echo '$null file is not null.'
else
echo '$null is null'
fi