Fix struct column with null add and insert

[Ulimo]

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Pull Request Overview

This PR aims to fix the handling of null values in struct columns during add and insert operations. The changes check for null values using value.IsNull and ensure that NullValue.Instance is added to each underlying column when null values are encountered.

• Modified Add method to check for null values and handle them appropriately.
• Modified InsertAt method to perform similar null value handling.

[github-actions]

Benchmark

Benchmark suite	Current: fdc7f1a	Previous: 22d4209	Ratio
FlowtideDotNet.Benchmarks.Stream.StreamBenchmark.InnerJoin	408941080 ns (± 9350724.694648597)	458826533.3333333 ns (± 10640413.19005047)	0.89
FlowtideDotNet.Benchmarks.Stream.StreamBenchmark.LeftJoin	514145577.7777778 ns (± 24304418.30890928)	564994640 ns (± 28550338.329818636)	0.91
FlowtideDotNet.Benchmarks.Stream.StreamBenchmark.ProjectionAndNormalization	157265560 ns (± 14717406.614406554)	169842820 ns (± 7144067.768715523)	0.93
FlowtideDotNet.Benchmarks.Stream.StreamBenchmark.SumAggregation	166598400 ns (± 17090261.711018678)	171581250 ns (± 12415362.812660772)	0.97
FlowtideDotNet.Benchmarks.Stream.StreamBenchmark.ListAggWithMapAggregation	1908074550 ns (± 151477018.23421744)	1982296640 ns (± 123349811.75190067)	0.96
FlowtideDotNet.Benchmarks.Stream.StreamBenchmark.WindowSum	350316650 ns (± 25310061.39644206)
FlowtideDotNet.Benchmarks.Stream.StreamBenchmark.ListAggWithStructAggregation	1451564566.6666667 ns (± 43693112.89948909)

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