Fixed per-frame-render warning in Firefox when series fillStyle undefined.

If a series does not specify a fillStyle, Firefox logs a warning each frame render:

> canvas: an attempt to set strokeStyle or fillStyle to a value that is neither a string, a CanvasGradient, or a CanvasPattern was ignored. @ http://localhost:8080/smoothie.js:301

This change fixes the issue by only setting the canvasContext.fillStyle property when seriesOptions.fillStyle is defined.