If the action has output which has not mandatory fields and none of them is present, the NetconfMessageTransformer reports error.
I found this problem to be present also for both the RPCs and actions.
Let's say we have two RPCs. One with output defined, but with non mandatory field and the second without output:
Regarding the Yang 1.1 RFC https://tools.ietf.org/html/rfc7950#section-7.14.4 there should be a possibility to return only "ok" message, when no output parameters are returned:
If the RPC operation invocation succeeded and no output parameters are returned, the <rpc-reply> contains a single <ok/> element defined in [RFC6241]. If output parameters are returned, they are encoded as child elements to the <rpc-reply> element defined in [RFC6241], in the same order as they are defined within the "output" statement.
So for both should be fine to get response:
But for the RPC with output defined "rpc-with-output" the NetconfMessageTransformer fails with error.