Add a way to distinguish between null and empty

[PujolDavid]

Following the issue: #469

I made a pull request with that proposal:

• Add a new feature EMPTY_UNQUOTED_STRING_AS_NULL
• Implement it like EMPTY_STRING_AS_NULL
• Add tests

Note that I made this PR on the branch 2.18, but I'm using the version 2.15 in my projects (Spring boot 3.1)
If accepted we can move this feature also to previous versions

[cowtowncoder]

Since this is API addition, SemVer would not allow us to backport it. In practical terms, it is unlikely new 2.15 versions will be released. But I could perhaps be convinced that merging into 2.17 (for 2.17.1) would be ok as "dark launch" (that is, officially feature is only available on 2.18 but in practice is included earlier if you know where to look).

[PujolDavid]

That would be great to have it on 2.17.1
Tell me if you want me to rebase on the 2.17 branch.

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After that, what will be the next step?
Will I need to do something else or it's now on your side?

[cowtowncoder]

I can cherry-pick change back to 2.17 if everything works, so 2.18 is fine.

But one other thing I need is CLA, from:

https://github.com/FasterXML/jackson/blob/master/contributor-agreement.pdf

and the usual way is to print it, fill & sign, scan/photo, email to cla at fasterxml dot com.
This only needs to be done once before the first contribution, and then is good for any and all contributions to Jackson components.

I hope to review this PR soon, hopefully today.

[PujolDavid]

Document sent, Thanks

[cowtowncoder]

All good with CLA; I have been busy but will try to get this reviewed and merged tomorrow.

[cowtowncoder]

Ok, so: this went officially in 2.18 for 2.18.0, but I did cherry-pick it for 2.17 branch: so technically it will go in as undocumented feature of 2.17.1 ("dark launch" so to speak).

[PujolDavid]

Great thanks a lot :)