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

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.

Start at src/producer.cc near line 559 and trace how the fifth opaque argument to produce() is checked and wrapped. Verify the null case does not retain a persistent handle, then run the relevant producer tests or add a focused regression test showing that passing null does not leak.

Written by the indexing model from the issue text.