Make generated Jackson serializers to work with null values of boxed types

[mariofusco]

As mentioned here, merging this commit actually unveiled a couple of defects in the generated Jackson serializers:

1. The getter method of a boolean field should start with "is" only when using the primitive type boolean, but not with the corresponding boxed type java.lang.Boolean. Fixed here.
2. When the Java object to be serialized has a boxed type field, the value of the field is automatically unboxed during the serialization, but of course this fails when the value is null, so it has to be guarded by a null check. Fixed here.

[quarkus-bot]

Status for workflow Quarkus CI

This is the status report for running Quarkus CI on commit acb52a9.

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It should be safe to merge provided you have a look at the other checks in the summary.

You can consult the Develocity build scans.

[gsmet]

@mariofusco could you also prepare a PR targeting 3.14? I suppose we need this fix in 3.14 too, right?

[mariofusco]

@mariofusco could you also prepare a PR targeting 3.14? I suppose we need this fix in 3.14 too, right?

Sure, I will immediately.

[gsmet]

Thanks!

[mariofusco]

@gsmet see #43157