webgl unbound buffer warning for empty non-directDraw StageGL

[adabru]

Using master branch (commit c7b6035) the following lines:

var stage = new createjs.StageGL("testcanvas", {directDraw:false})
stage.update()

create this webgl warning (Ubuntu 16.04, Chrome 69.0.3497.42 beta 64-bit):

WebGL: INVALID_OPERATION: drawArrays: no buffer is bound to enabled attribute
    p._renderCover @ easeljs-NEXT.js:11763
    p._drawCover @ easeljs-NEXT.js:11389
    p.draw @ easeljs-NEXT.js:10251
    p.update @ easeljs-NEXT.js:10182

If I add something renderable to the scene, the warning doesn't appear:

var shape = new createjs.Shape()
shape.cache(0,0,50,50)
stage.addChild(shape)
// this update call won't cause a webgl warning anymore
stage.update()

https://codepen.io/adabru/pen/OoPaLN/?editors=1000

The affected code is in the functions StageGL@_renderCover and StageGL@_renderBatch I guess. In these functions some gl.bindBuffer, gl.vertexAttribPointer and gl.bufferSubData are called. If I add the gl functions from _renderBatch inside of _renderCover, the warning also disappears. I guess that is the reason why the warning doesn't appear when the stage has some children.

As workaround I add an empty, useless shape to the scene.

[DavidHGillen]

The warning is mostly harmless. I debated marking this is "won't fix", but given the way I constantly suggest people track the rate of renders per draw it seemed workable to track that and avoid the warning for everyone's sake.