how to check the string is Null or not in shell script?

I had a script like this

    #!/bin/bash
    line="hello"

    if [ -z $line ] ; then
            echo "String null"
    fi

This will work properly, but when I give the lineas follows

    line="hello welcome"

It will through the error as

     a.sh: 5: [: hello: unexpected operator

In that situation , how can I check that is Null or no ??