Fix for warning "an attempt to set strokeStyle or fillStyle to a value that is neither a string, a CanvasGradient, or a CanvasPattern was ignored"

In this example http://iioengine.com/docs/iio-basics#this box shadow is an object that has no fillStyle and strokeStyle properties defined. Because of this Firefox emits warning "context: an attempt to set strokeStyle or fillStyle to a value that is neither a string, a CanvasGradient, or a CanvasPattern was ignored" in lines 1044, 1045 on each draw cycle. This slow downs animation and pollutes firebug console.
In Chromium there is no warning.