Add example of unintentional None to 2.19 porting guide

[felixfontein]

Ref: ansible/ansible#85605

[felixfontein]

The result can be seen here: https://ansible--2982.org.readthedocs.build/projects/ansible-core/2982/porting_guides/porting_guide_core_2.19.html#example-unintentional-none

[samccann]

@felixfontein @mattclay - looks like the referenced issue suggests some of this is updated in the next core 2.19 version. Is the content in this PR update still valid?

[felixfontein]

I was planning to check whether the PR mentioned there (ansible/ansible#85652) changed something, but only got to do that now. It actually does (the first string_with_none now is a stringified list without the Nones). Since according to ansible/ansible#85605 (comment) the new behavior isn't fixed it's better to wait until that's finalized.

CC @nitzmahone please add a comment here if/when you decide to change something :)

[mattclay]

The fixes in ansible/ansible#85652 and ansible/ansible#85676 resolve the issue with concatenation of None resulting in a literal None in the template output. These fixes are being backported (ansible/ansible#85663) and will be included in 2.19.1.

[felixfontein]

I'll rewrite this accordingly somewhen this weekend.