fix: Validation cannot handle array item

[kenjis]

Description
Supersedes #5402
Fixes #5368

• separate validation process of multiple field and single field

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[MGatner]

The code is a bit hard to review on mobile but what I see all looks good, and the tests are great.

Your User Guide write-up is very thorough, but I would rather see that in the changelogs (i.e. user_guide_src/source/changelogs/v4.1.6.rst) and keep the Upgrading section as actual tasks that need to be done when upgrading. A simple sentence with reference to the longer explanation in the changelog would be good, like "Due to a bug fix Validation now might ____ (see ____.rst), so make sure you ____ to any code calling it."

[MGatner]

Will this conflict with the Strict Validation changes?

[kenjis]

@MGatner I added the last commit to fix the docs. I rebased because the commits are not verified.

Will this conflict with the Strict Validation changes?

No.
#5402 will be split into two PRs. This is one.

[MGatner]

Doc changes look good! Since this is a breaking change I'd like it if you could get another reviewer. I'll ping the team on Slack.

[paulbalandan]

Since this one is the same with L145, I think we can combine this with that?

[kenjis]

If I remove the if statement from L143, tests in tests/system/Validaton/ passes.
But I'm not sure removing it causes no problem.

What input makes $values empty array?

[kenjis]

I added continue in the if to fix the bug that $this->processRules() run twice.

[paulbalandan]

What input makes $values empty array?

This is raised in an issue report. I'll check.

[paulbalandan]

#4510 (comment)

[kenjis]

I read the comment and the fix PR.
But I'm not sure we can remove the if ($values === []) { ... }.
I feel it is safer not to remove it.

[kenjis]

@paulbalandan I want to update upgrade_416.rst, so I merge this for now.