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.

Example:

Let's say we have two RPCs. One with output defined, but with non mandatory field and the second without output:

rpc rpc-with-output {
   output {
      leaf not-mandatory-message {
         type string;
      }
   }
}

rpc rpc-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:

<rpc-reply xmlns="urn:ietf:params:xml:ns:netconf:base:1.0">
   <ok/>
</rpc-reply>

But for the RPC with output defined "rpc-with-output" the NetconfMessageTransformer fails with error.