fix(flow): deal with flow drop leftover

[discord9]

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Refer to a related PR or issue link (optional)

What's changed and what's your intention?

previously, if a flow& it's sink table is dropped, but in flownode's output buf there is still leftover for this flow, calling flush_flow here will cause a bug which when send insert request back, can't found the sink table(as expected because sink table is dropped)
this is fixed now by check if table no longer exist and not make send back request failed
related failed test: https://github.com/GreptimeTeam/greptimedb/actions/runs/12825330060/job/35763543123

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

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