writeMeta function
Writes _meta into json only when non-null and non-empty.
Avoids serializing _meta: null or _meta: {}, matching the schema's
nullable-with-default-null semantics.
Implementation
void writeMeta(Map<String, Object?> json, Map<String, Object?>? meta) {
if (meta != null && meta.isNotEmpty) {
json['_meta'] = meta;
}
}