Add zero-length deserialize unit test.

PiperOrigin-RevId: 606637997

rust/test/shared/serialization_test.rs

@@ -8,6 +8,13 @@
 use googletest::prelude::*;
 use unittest_proto::TestAllTypes;
 
+#[test]
+fn serialize_zero_length() {
+    let msg = TestAllTypes::new();
+    let serialized = msg.serialize();
+    assert_that!(serialized.len(), eq(0));
+}
+
 #[test]
 fn serialize_deserialize_message() {
     let mut msg = TestAllTypes::new();