BooleanWeight should not use BS1 when there is a single non-null clause

When a disjunction has a single non-null scorer, we still use BS1 for bulk-scoring, which first collects matches into a bit set and then calls the collector. This is inefficient: we should just call the inner bulk scorer directly and wrap the scorer to apply the coord factor (like BooleanTopLevelScorers.BoostedScorer does).