fix Python bug with required fields

https://github.com/protocolbuffers/upb/issues/1220

There were two bugs here: Python was incorrectly mandating that a required
field be set during assignment, and it was also incorrectly assuming a non-NULL
return from an internal function call.

PiperOrigin-RevId: 518561818

python/google/protobuf/internal/message_test.py

@@ -53,6 +53,7 @@ cmp = lambda x, y: (x > y) - (x < y)
 from google.protobuf.internal import api_implementation # pylint: disable=g-import-not-at-top
 from google.protobuf.internal import encoder
 from google.protobuf.internal import more_extensions_pb2
+from google.protobuf.internal import more_messages_pb2
 from google.protobuf.internal import packed_field_test_pb2
 from google.protobuf.internal import test_proto3_optional_pb2
 from google.protobuf.internal import test_util
@@ -496,6 +497,13 @@ class MessageTest(unittest.TestCase):
     msg.repeated_nested_message.MergeFrom(other_msg.repeated_nested_message)
     self.assertEqual([1, 2, 3, 4], [m.bb for m in msg.repeated_nested_message])
 
+  @unittest.skipIf(
+      api_implementation.Type() == 'upb',
+      "We can enable after protobuf updates to use the current upb repo")
+  def testInternalMergeWithMissingRequiredField(self, message_module):
+    req = more_messages_pb2.RequiredField()
+    more_messages_pb2.RequiredWrapper(request=req)
+
   def testAddWrongRepeatedNestedField(self, message_module):
     msg = message_module.TestAllTypes()
     try:

python/google/protobuf/internal/more_messages.proto

@@ -358,3 +358,11 @@ message LotsNestedMessage {
   message B254 {}
   message B255 {}
 }
+
+message RequiredField {
+  required int32 id = 1;
+}
+
+message RequiredWrapper {
+  optional RequiredField request = 1;
+}