[TIMOB-25255] Windows: Setting label.color to null causes label text to become transparent

Description

Setting color property of a label to null causes the label text to become transparent, it should go to the default colour for the platform, I have not tested any other UI elements

var _window = Ti.UI.createWindow();
var label = Ti.UI.createLabel({
    text: 'just a test label'
});
label.addEventListener('click', function(e) {
    label.color = null;
});
_window.add(label);
_window.open()

Steps to reproduce

Add the above code to an existing app.js

Tap the label

Actual

Label becomes transparent

Expected

Label should go to default color