Add error handling when misuse of find() with array values

[symfonyaml]

Q	A
Branch?	3.4.x
Bug fix?	yes
New feature?	no
Issues	Fix #11236

Issue

When we (inexpertly) use $em->find(...) with array values in identifer, it returns a PHP warning..
See all details and how to reproduce it in the issue : #11236.

Solution

In this PR

• Add some DX with an exception explaining the problem, to avoid PHP warning
• Add tests

Notes

This PR is a copy of this PR who has been re-targeted to branch 3.1.x

[github-actions]

There hasn't been any activity on this pull request in the past 90 days, so it has been marked as stale and it will be closed automatically if no further activity occurs in the next 7 days.
If you want to continue working on it, please leave a comment.

[github-actions]

There hasn't been any activity on this pull request in the past 90 days, so it has been marked as stale and it will be closed automatically if no further activity occurs in the next 7 days.
If you want to continue working on it, please leave a comment.

[greg0ire]

Please kindly squash your commits together. If you don't, we'll try to remember to do it for you but it's best if you save us this trouble.

How to do that?

1. git rebase -i origin/3.4.x, assuming origin is a git remote that points to this repository, and not your fork. If you're not sure what your remotes are, run git remote -vvv, there should be your fork and the holy/reference/base/origin/whatever-you-call-it repository.
2. A window will show up with many lines, replace pick with fixup on every line but the first one
3. Close your editor, git should do its magic, and you should end up with one commit
4. Use git push --force to overwrite what you already pushed. Don't forget the --force option otherwise git will try to merge both things together.

[symfonyaml]

@greg0ire I rebased the PR as you suggested. But now I have this BackedEnum check coming from 3.4.x.

    foreach ($identifier as $k => $value) {
        if ($value instanceof BackedEnum) {
            $identifier[$k] = $value->value;
        }
    }

I thought you said it was redundant so I am confused...

[greg0ire]

In the interactive rebase window, you can drop any commit you feel is unnecessary… I don't know how you managed to make them appear.