Don't emit code for empty files

[nirinchev]

#1438 introduced a regression where we would emit // ignore_for_file: type=lint for every file, even those that don't have any realm models, resulting in warnings like:

xyz.g.dart must be included as a part directive in the input library with:
part 'xyz.g.dart';

for files that don't contain models and should not have needed part files.