INVALID_VALUE: texImage2D: no canvas

[jamesleesaunders]

Fix bug described on x3dom/x3dom#910

11WebGL: INVALID_VALUE: texImage2D: no canvas
245[.WebGL-0x7fc3bc0dd000]RENDER WARNING: texture bound to texture unit 0 is not renderable. It maybe non-power-of-2 and have incompatible texture filtering.
BarChartMultiSeries.html:1 WebGL: too many errors, no more errors will be reported to the console for this context.

@andreasplesch

I did some tracing using the dev tools debugger.
It seems that a text node is generated for the axis labeling with an empty string attribute:

This causes then nothing to be rendered on the canvas which is used as a texture for the text which caueses the console error. It should be harmless but probably worth checking why the no content text node is generated in the first place.