feat(spring): enable `openApiNullable` with `useJackson3` #108

Job	Run time
Build Java Helidon (samples/client/petstore/java-helidon-client/v3/se, 17)	1m 3s
Build Java Helidon (samples/server/others/java-helidon-server/v3/mp-format-test, 17)	59s
Build Java Helidon (samples/server/petstore/java-helidon-server/v3/se, 17)	45s
Build Java Helidon (samples/server/petstore/java-helidon-server/v3/mp, 17)	56s
Build Java Helidon (samples/client/petstore/java-helidon-client/v3/mp, 17)	1m 2s
	4m 45s

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