Properly serialize zero-value messages in a oneof group (#176)

Also improve test utils to make it easier to have multiple json examples.

Co-authored-by: Christopher Chambers <chris@peanutcode.com>

tests/inputs/oneof_empty/oneof_empty.proto

@@ -0,0 +1,15 @@
+syntax = "proto3";
+
+message Nothing {}
+
+message MaybeNothing {
+  string sometimes = 42;
+}
+
+message Test {
+  oneof empty {
+    Nothing nothing = 1;
+    MaybeNothing maybe1 = 2;
+    MaybeNothing maybe2 = 3;
+  }
+}