Enforce serialize_empty for repeated fields (#417)

tests/inputs/regression_387/test_regression_387.py

@@ -0,0 +1,12 @@
+from tests.output_betterproto.regression_387 import (
+    ParentElement,
+    Test,
+)
+
+
+def test_regression_387():
+    el = ParentElement(name="test", elems=[Test(id=0), Test(id=42)])
+    binary = bytes(el)
+    decoded = ParentElement().parse(binary)
+    assert decoded == el
+    assert decoded.elems == [Test(id=0), Test(id=42)]