Fix unreleased regression where processors using getPaymentDescription get a fatal

Fix unreleased regression where processors using getPaymentDescription get a fatal

We added the new property bag in https://github.com/civicrm/civicrm-core/pull/15697 but in the back & forth the
call to the function getDescription was retained but the function was removed. This would leave the
core processor that calls it (Paypal std) with a fatal. The known external processor using it (Omnipay)
also hits a notice because the signature changed ($params became optional). From the Omnipay
pov I think it makes sense just to stop overriding the function - hence I am sneaking a tiny
change into this to make that possible.

Note the doPayment function ensures that propertyBag is set. There is a small chance that an external processor
is fully overriding doPayment AND calling this - that would be the case for Omnipay except I'll remove
the override in conjunction with this - so I've added a few lines to ensure it is set.

Note the test would fail with the changes to Payment_Dummmy & not the changes to Core_Payment so it provides
test cover