Memory Leak (global handles) when passing null as opaque argument to produce()

[lassic]

https://github.com/Blizzard/node-rdkafka/blob/master/src/producer.cc#L559

We're seen a memory leak passing null as the opaque argument to produce.
This was subtle and tough to debug, and is quite surprising behavior.
I think the 5th argument check should also include !IsNull much like the 4th argument.
Otherwise, global (persistent) handles are leaked wrapping a null value, which doesn't make sense.