Fill in defaults for omitted parameters to RecentChange::sendToUDP

Not all callers of RecentChange::sendToUDP pass all the parameters -- which is
to be expected, since the sendToUDP's signature specifies the empty string as
default. It is not correct to leave the values empty, though, since they are
used to construct the logging URL. When the parameters are unset, the correct
behavior should be to fill in defaults from $wgRC2UDP*, as was the case prior
to I270bde418. This patch reverts to the old, correct behavior.

Change-Id: I4570473ae7382e602292e58a41e440325a65a275
(cherry picked from commit 23254c1eca50ca1929852c9f8a5c97723b911f04)