Provide a blank default global layout option

[etimberg]

Resolves #4309

[simonbrunel]

I think it's better to expand the default option to the object form to get a better options merge?

// defaults:
layout: {
    padding: {
        top: 0,
        right: 0,
        bottom: 0,
        left: 0
    }
}

// merged with:
layout: {
    padding: {
        top: 16
    }
}

// produces:
layout: {
    padding: {
        top: 16,
        right: 0,
        bottom: 0,
        left: 0
    }
}

// instead of:
layout: {
    padding: {
        top: 16
        //right: undefined
        //bottom: undefined
        //left: undefined
    }
}

[etimberg]

Sure, that's a good idea. Will update