Enable `StrictNullChecks` by default

[k163377]

Previously, the StrictNullChecks option was disabled by default, which compromised null safety.
Therefore, I propose to enable this option by default in 3.0.

As a side note, since 2.19, improvements in #885 have significantly reduced the throughput degradation when the StrictNullChecks option is enabled, and the deserialization performance is almost the same as when it is disabled.
Thus, performance concerns have at least been addressed.

Please vote with 👍 or 👎.

[cowtowncoder]

@k163377 I'll update https://github.com/FasterXML/jackson-future-ideas/wiki/JSTEP-2 and add an entry or two from here. But going forward maybe you can add links from there?

[k163377]

@cowtowncoder
Thank you, I will do so.

[k163377]

The change have been reflected (sorry for not making it to RC1 in time).