Return the port element of this CellView that is identified by portId (i.e. the SVGElement within this.el which has 'port': portId).

If the CellView does not have a port identified by portId, return null.

cellView.findPortNode(portId, selector)

If selector is also specified, return the subelement of the port element that is identified by selector, according to the rules of the cellView.findBySelector() function.

If the port element does not have a subelement that matches selector, return null.