[ox] Fix Mac build and fix MC to increment field counter on empty string

(synced from 819959745d)

src/ox/mc/read.hpp

@@ -225,12 +225,13 @@ Error MetalClawReader::field(const char*, Handler handler) {
 template<typename T>
 Error MetalClawReader::field(const char* name, ox::Vector<T> *val) {
 	if (m_unionIdx == -1 || m_unionIdx == m_field) {
+		// set size of val if the field is present, don't worry about it if not
 		if (m_fieldPresence.get(m_field)) {
 			const auto [len, err] = arrayLength(name, false);
 			oxReturnError(err);
 			val->resize(len);
-			return field(name, val->data(), val->size());
 		}
+		return field(name, val->data(), val->size());
 	}
 	return OxError(0);
 }