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.

[Diamondlord]

We also confirmed the memory leak occurs when an empty string ("") is passed for the opaque argument. This was discovered because opaque is before the headers argument we use for tracing propagation.