ThrowableDeserializer does not handle null well for cause

[cowtowncoder]

Describe your Issue

Currently JSON for deserializing any Throwable with null for "cause" property:

{ "message": "...",
   "cause": null
}

will lead to cryptic exception, due to Throwable.initCause() being called with null.
To avoid this, ThrowableDeserializer should handle this edge case gracefully.