fix: resolve Miri UB in null struct field test, re-enable Miri on PRs #6016

Job	Run time
Lint	42s
Build Native Library (macOS)	3m 35s
macos-14/Spark 3.4, JDK 11, Scala 2.12 [exec]	18m 49s
macos-14/Spark 3.4, JDK 11, Scala 2.12 [shuffle]	27m 51s
macos-14/Spark 3.4, JDK 11, Scala 2.12 [csv]	8m 23s
macos-14/Spark 3.4, JDK 11, Scala 2.12 [parquet]	15m 44s
macos-14/Spark 3.4, JDK 11, Scala 2.12 [sql]	8m 42s
macos-14/Spark 3.4, JDK 11, Scala 2.12 [fuzz]	29m 57s
macos-14/Spark 3.5, JDK 17, Scala 2.13 [shuffle]	20m 26s
macos-14/Spark 3.5, JDK 17, Scala 2.13 [fuzz]	32m 57s
macos-14/Spark 3.5, JDK 17, Scala 2.13 [parquet]	19m 30s
macos-14/Spark 4.0, JDK 17, Scala 2.13 [csv]	8m 4s
macos-14/Spark 3.5, JDK 17, Scala 2.13 [expressions]	22m 39s
macos-14/Spark 3.4, JDK 11, Scala 2.12 [expressions]	28m 13s
macos-14/Spark 4.0, JDK 17, Scala 2.13 [expressions]	20m 22s
macos-14/Spark 4.0, JDK 17, Scala 2.13 [exec]	13m 57s
macos-14/Spark 4.0, JDK 17, Scala 2.13 [shuffle]	19m 33s
macos-14/Spark 3.5, JDK 17, Scala 2.13 [exec]	17m 58s
macos-14/Spark 4.0, JDK 17, Scala 2.13 [parquet]	12m 22s
macos-14/Spark 4.0, JDK 17, Scala 2.13 [sql]	11m 19s
macos-14/Spark 3.5, JDK 17, Scala 2.13 [csv]	10m 17s
macos-14/Spark 4.0, JDK 17, Scala 2.13 [fuzz]	20m 11s
macos-14/Spark 3.5, JDK 17, Scala 2.13 [sql]	8m 20s
	6h 19m 51s

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