feat(codegen): improve nullability of generated types #2313

Job	Run time
all-platforms (ubuntu-latest)	20m 34s
all-platforms (macos-latest)	27m 39s
all-platforms (windows-latest)	29m 37s
jvm (8)	7m 12s
jvm (11)	6m 20s
jvm (17)	8m 5s
jvm (21)	7m 30s
	1h 46m 57s

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