Support field nullability information in stream closures for final fields

NullAway throws an error [NullAway] dereferenced expression allowedNames is @Nullable at .filter(name -> allowedNames.contains(name)). I was not expecting this error because of the explicit null check for allowedNames.

import java.util.stream.Collectors;
import java.util.Collections;
import java.util.List;
import java.util.Set;
import javax.annotation.Nullable;

 class TestStream{

    @Nullable private final Set<String> allowedNames;
    TestStream(@Nullable Set<String> allowedNames) {
      this.allowedNames = allowedNames;
    }

    private List<String> filterAllowedNames(List<String> inputNames){
      if (allowedNames == null) {
        return inputNames;
      }
      if (allowedNames.isEmpty()) {
        return Collections.emptyList();
      }
      return inputNames
              .stream()
              .filter(name -> allowedNames.contains(name))
              .collect(Collectors.toList());

    }
  }

The issue provides a Java TestStream reproducer in the report; start by running it through NullAway and confirming the false-positive dereference warning inside the stream filter closure. Trace how the analyzer carries nullability for the final allowedNames field into closures. Done means the explicit null check is respected and the reproducer no longer reports an error.