code: private @Nullable CodeBlock initialValue(Field field) {
      if (field.isPacked() || field.isRepeated()) {
        return CodeBlock.of("$T.newMutableList()", Internal.class);
      } else if (field.type().isMap()) {
        return CodeBlock.of("$T.newMutableMap()", Internal.class);
      } else {
        return null;
      }
    }
docs: Returns the initial value of {@code field}, or null if it is doesn't have one.