chore: remove merge's explicit nullability (#4967) #3647

Job	Run time
bench (compress, Compression)	24m 58s
bench (random_access, Random Access)	7m 44s
commit-metadata	26s
sql / bench (tpch-nvme, tpch, TPC-H SF=1 on NVME, datafusion:arrow,datafusion:parquet,datafusion:vortex...	12m 8s
sql / bench (clickbench-nvme, clickbench, Clickbench on NVME, datafusion:parquet,datafusion:vortex,data...	54m 44s
sql / bench (fineweb, fineweb, FineWeb, bench-vortex/data/fineweb, datafusion:parquet,datafusion:vortex...	9m 12s
sql / bench (tpch-s3, tpch, TPC-H SF=1 on S3, bench-vortex/data/tpch/1.0, s3://vortex-bench-dev-eu/deve...	15m 40s
sql / bench (tpcds-nvme, tpcds, TPC-DS SF=1 on NVME, datafusion:parquet,datafusion:vortex,datafusion:vo...	7m 10s
sql / bench (tpch-nvme-10, tpch, TPC-H SF=10 on NVME, datafusion:arrow,datafusion:parquet,datafusion:vo...	22m 29s
sql / bench (tpch-s3-10, tpch, TPC-H SF=10 on S3, bench-vortex/data/tpch/10.0, s3://vortex-bench-dev-eu...	28m 47s
sql / bench (statpopgen, statpopgen, Statistical and Population Genetics, bench-vortex/data/statpopgen,...	31m 26s
	3h 34m 44s

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