perf(streaming): optimzie topN cache

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When the streaming TopN cache are deleted to no records, RW will try to scan from the table and fill the cache. But if there is no records in the table or group, TopN cache will scan from the table again and again. It is especially bad when the table has many delete tombstones.
The issue appeared in the groupTopN, the Agg's minput should have been solved with the row count.

@Little-Wallace found in NexMark q9 with temporal filter