add omitempty to mutator's type & eval fields (#4433)

* A little more efficient but also will help clients determine whether they should fallback to the default value.
* If we go ahead with this, it might be best to do so before the v6.5 release, otherwise may be considered a breaking change.
* I had issues with generating the structs from the proto; may need someone else to look at it.

---

Signed-off-by: James Phillips <jamesdphillips@gmail.com>
Co-authored-by: Cyril Cressent <cyril@sensu.io>

api/core/v2/mutator.proto

@@ -45,9 +45,9 @@ message Mutator {
   // and run on the Otto VM. The runtime assets will be assumed to be javascript
   // assets, and the environment variables will be made available to the global
   // environment of the mutator.
-  string type = 10 [ (gogoproto.jsontag) = "type" ];
+  string type = 10 [ (gogoproto.jsontag) = "type,omitempty" ];
 
   // When the type of the mutator is "javascript", the eval field will be expected
   // to hold a valid ECMAScript 5 expression.
-  string eval = 11 [ (gogoproto.jsontag) = "eval" ];
+  string eval = 11 [ (gogoproto.jsontag) = "eval,omitempty" ];
 }