string1 has not been initialized, it has no defined value

1. Non-quote string in test brackets. Wrong results !
> [ -n $string1 ] &&  echo "String1 is not null"
-> Show "String1 is not null" --> Wrong results.
2. Quote string within test brackets.
> [ -n "$string1"] && echo "String1 is not null"
->    --> This works fines
3. string1 stands naked. Must quoting. See the following examples
    3.1 Unquoted
> [ $string1 ] && echo "String1 is not null"
->   --> This works fine.
> string1="a = b"
> [ $string1 ] && echo "String1 is not null"
->   --> Wrong result !
    3.2 Quoted
> string1="a = b"
> [ "$string1" ] && echo "String1 is not null"
-> Show "String1 is not null"  --> This works fine