do not cast to JSON explicitly #20938

Job	Run time
flow_test (ubuntu-latest-16-cores, 17, mysql-pos, 7.0, stable)	21m 43s
flow_test (ubuntu-latest-16-cores, 16, mysql-gtid, 6.0, lts)	19m 49s
flow_test (ubuntu-latest-16-cores, 18, mysql-gtid, 8.0, latest)	19m 30s
	1h 1m 2s

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