use allowEmpty at showPaletteOnly

[Ji-yeonPark]

Hi there.

There was solution for this function at #473 , but that was not work for me.
so, I fixed small error about "showPaletteOnly & allowEmpty".

✈️ HOW TO USE -1
This code can use allowEmpty option at "showPaletteOnly : true".

• set allowEmpty to true
• Put the '' at palette where you want it.

$("#shadow").spectrum({
        showPaletteOnly: true,
        allowEmpty: true,
        palette: [
            ['', 'black', 'white'],
            ['green', 'blue', 'yellow']
        ]
})

✈️ RESULT

✈️ HOW TO USE -2
it can be used "allowEmpty" option normally.

• set allowEmpty to true

$("#shadow").spectrum({
        allowEmpty: true,
        color: "#ff0000",
})

✈️ RESULT