Jupyterlab-phosphor should not attempt to load empty array

[timkpaine]

https://github.com/finos/perspective/blob/master/packages/perspective-phosphor/src/ts/widget.ts#L319

This code should be

if (Array.isArray(data)) 
    if (data.length > 0{
        if (this._wrap) {
            this.pspNode.update([data]);
        } else {
            this.pspNode.update(data);
        }
    } else {
        console.warn('Perspective received length 0 data');
    }
} else {
...