message: Fixes rendering when graph dimensions are null
  This happened when sigma tried to render a graph with just one node.
diff: @@ -1090,8 +1090,8 @@ export default class Sigma extends EventEmitter {
  const extent = this.customBBox || this.nodeExtent;
  return {
  - width: extent.x[1] - extent.x[0],
  - height: extent.y[1] - extent.y[0],
  + width: extent.x[1] - extent.x[0] || 1,
  + height: extent.y[1] - extent.y[0] || 1,
  };
  }
  