[TIMOB-14575] iOS: setting borderRadius to null causes error

problem

Setting *borderRadius* to null causes the targeted proxy to disappear on iOS. I'm not entirely sure what is happening WRT to the borderRadius to cause the proxy to disappear entirely. On Android and Mobileweb the borderRadius appears to be simply set to zero as a developer would expect.

expected

Setting borderRadius to null should remove the borderRadius (or set it to 0) on all platforms without error.

test case

var win = Ti.UI.createWindow({
	backgroundColor: '#fff',
	modal: false,
	exitOnClose: true
});
var label = Ti.UI.createLabel({
	text: 'just a test label',
	borderColor: '#0f0',
	borderWidth: 2,
	borderRadius: 8
});
label.addEventListener('click', function(e) {
	label.borderRadius = null;
});

win.add(label);
win.open();