Not a few user use user defined "Extended Operator" and some of them code "return null."
Because there are many samples using "return null" in "Extensions Guide" for 5.5 or prior version, even if "Guide to Creating Corticon Extensions" for 5.6 has a few sample using "return null".

However, your technical support member advised me that "An Extended Operator should not return a null value" in my support case.

Hence I suppose it must be better to change the sample code not to use "return null" in each documentation and to inform it via article, and so on.