[5.5] Adds default string value to Artisan line method

[nunomaduro]

This PR gives the possibility of put an empty line on an Artisan command without having to put an empty string as parameter.

Before:

        $this->line(''); // <-- We have to put an empty string in order to add a new empty line.

After:

        $this->line();
    }

The result is still the same, an empty line:

[GrahamCampbell]

I think 5.6 is the best place for this tbh.

[carusogabriel]

@nunomaduro There's a method call newLine

[taylorotwell]

Indeed, see newLine method :)

[nunomaduro]

@carusogabriel @taylorotwell Thanks! 👍