Fix serialization of repeated fields with empty messages (#180)

Extend test config and utils to support exclusion of certain json samples from
testing for symetry.

tests/inputs/empty_repeated/empty_repeated.json

@@ -0,0 +1,3 @@
+{
+  "msg": [{"values":[]}]
+}

tests/inputs/empty_repeated/empty_repeated.proto

@@ -0,0 +1,9 @@
+syntax = "proto3";
+
+message MessageA {
+  repeated float values = 1;
+}
+
+message Test {
+  repeated MessageA msg = 1;
+}