fix _slice9 == [] (#256)

* fix _slice9 == []

[] == [] is false, because arrays
i simply changed `_slice9 == []` to `_slice9.length != 4` which would achieve the intended purpose, i think

* warn if given invalid 9slice array

how can you do this with only 1 conditional again

* Update flixel/addons/ui/FlxUI9SliceSprite.hx

Co-authored-by: George Kurelic <Gkurelic@gmail.com>

---------

Co-authored-by: George Kurelic <Gkurelic@gmail.com>

flixel/addons/ui/FlxUI9SliceSprite.hx

@@ -158,8 +158,10 @@ class FlxUI9SliceSprite extends FlxUISprite implements IResizable implements IFl
 		var iw = Std.int(pt.x);
 		var ih = Std.int(pt.y);
 
-		if (_slice9 == null || _slice9 == [])
+		if (_slice9 == null || _slice9.length != 4)
 		{
+			if (_slice9 != null)
+				flixel.FlxG.log.warn("Invalid 9slice array, expected a length of 4");
 			_slice9 = [4, 4, 7, 7];
 		}