Fix default list type for frozen messages

[osa1]

Without the fix the test fails with

check read-only default list type [E]
  type '() => int' is not a subtype of type '(() => Never)?' of 'orElse'
  dart:collection                                    ListMixin.firstWhere
  package:protobuf/src/protobuf/pb_list.dart 260:20  PbListBase.firstWhere
  test/repeated_field_test.dart 40:12                main.<fn>

This is because the expression const [] has type List<Never> in
_FieldSet._getDefaultList.

[sigurdm]

LGTM