Return empty vertex data if there are no arrows
Return empty vertex data if there are no arrows

This would otherwise result in an error.

RebeccaWPerry/vispy,inclement/vispy,RebeccaWPerry/vispy,julienr/vispy,RebeccaWPerry/vispy,jdreaver/vispy,Eric89GXL/vispy,drufat/vispy,srinathv/vispy,inclement/vispy,srinathv/vispy,bollu/vispy,drufat/vispy,dchilds7/Deysha-Star-Formation,srinathv/vispy,inclement/vispy,ghisvail/vispy,jay3sh/vispy,kkuunnddaannkk/vispy,bollu/vispy,sbtlaarzc/vispy,jdreaver/vispy,jdreaver/vispy,sbtlaarzc/vispy,michaelaye/vispy,michaelaye/vispy,jay3sh/vispy,michaelaye/vispy,QuLogic/vispy,julienr/vispy,kkuunnddaannkk/vispy,Eric89GXL/vispy,QuLogic/vispy,julienr/vispy,QuLogic/vispy,drufat/vispy,dchilds7/Deysha-Star-Formation,dchilds7/Deysha-Star-Formation,ghisvail/vispy,jay3sh/vispy,sbtlaarzc/vispy,ghisvail/vispy,kkuunnddaannkk/vispy,Eric89GXL/vispy,bollu/vispy