Add an a = b, b > 0 example for dp_volumeFlowRate pressure loss model

[casella]

#4552 was fixed by #4553. We should add a test with a = 0 and b > 0 to ModelicaTest.Fluid.Dissipation.Verifications.PressureLoss.General.dp_volumeFlowRate_DPMFLOW that covers this specific case