Fix serialization of repeated fields with empty messages

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

src/betterproto/__init__.py

@@ -307,6 +307,7 @@ def encode_varint(value: int) -> bytes:
 
 def _preprocess_single(proto_type: str, wraps: str, value: Any) -> bytes:
     """Adjusts values before serialization."""
+
     if proto_type in [
         TYPE_ENUM,
         TYPE_BOOL,
@@ -738,9 +739,18 @@ def __bytes__(self) -> bytes:
                     output += _serialize_single(meta.number, TYPE_BYTES, buf)
                 else:
                     for item in value:
-                        output += _serialize_single(
-                            meta.number, meta.proto_type, item, wraps=meta.wraps or ""
+                        output += (
+                            _serialize_single(
+                                meta.number,
+                                meta.proto_type,
+                                item,
+                                wraps=meta.wraps or "",
+                            )
+                            # if it's an empty message it still needs to be represented
+                            # as an item in the repeated list
+                            or b"\n\x00"
                         )
+
             elif isinstance(value, dict):
                 for k, v in value.items():
                     assert meta.map_types

tests/inputs/config.py

@@ -21,3 +21,10 @@
     "example_service",
     "empty_service",
 }
+
+
+# Indicate json sample messages to skip when testing that json (de)serialization
+# is symmetrical becuase some cases legitimately are not symmetrical.
+# For tuple in the set, the first value is the name of the test and the second is
+# the name of the json file.
+non_symmetrical_json = {("empty_repeated", "empty_repeated")}

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;
+}