Fix CCLabel Debug.Assert when text is null and updating color with CCQuadCommand.RequestUpdateQuads.  There is no need to update the cascaded color if there is no TextureAtlas vertices to be updated.
Fix CCLabel Debug.Assert when text is null and updating color with CCQuadCommand.RequestUpdateQuads.  There is no need to update the cascaded color if there is no TextureAtlas vertices to be updated.

MSylvia/CocosSharp,mono/CocosSharp,MSylvia/CocosSharp,haithemaraissia/CocosSharp,zmaruo/CocosSharp,haithemaraissia/CocosSharp,TukekeSoft/CocosSharp,mono/CocosSharp,TukekeSoft/CocosSharp,zmaruo/CocosSharp