[BUG] List of message types shouldn't have nullable child

**Describe the bug**
`repeated MyMessage my_messages = 1;`

This generates a field with type `List<MyMessage?>`, however it should be `List<MyMessage>`.

**To Reproduce**
Code to reproduce the problem

```proto
message InvalidMessage { 
  repeated InvalidMessage invalidMessage = 1;
}
```

**Expected behavior**
`List<MyMessage>`