fix for STRUCT() with unknown fields in duckdb #54

Job	Run time
check-permission	2s
db-mysql / test-mysql	4m 30s
db-duckdb-wasm / test-duckdb-wasm	6m 54s
db-duckdb / test-duckdb	7m 5s
db-postgres / test-postgres	4m 7s
db-presto / test-presto	7m 37s
db-bigquery / test-bigquery	5m 53s
db-motherduck / test-motherduck	3m 54s
db-trino / test-trino	6m 33s
main / main	6m 34s
db-snowflake / test-snowflake	5m 39s
malloy-tests	0s
	58m 48s

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