…codes (xabbuh)

This PR was merged into the 4.4 branch.

Discussion
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[Validator] deprecate non-string constraint violation codes

| Q             | A
| ------------- | ---
| Branch?       | 4.4
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | yes
| Tests pass?   | yes
| Fixed tickets | #32066 (comment)
| License       | MIT
| Doc PR        |

Commits
-------

e217729 deprecate non-string constraint violation codes

This PR was merged into the 5.0-dev branch.

Discussion
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[Validator] Fix tests

| Q             | A
| ------------- | ---
| Branch?       | master <!-- see below -->
| Bug fix?      | yes
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | relates to #32265   <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | N/A

Fixes the following tests currently failing on master, as well as a bug where a null code was printed:

```diff
1) Symfony\Component\Validator\Tests\ConstraintViolationListTest::testToString
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
 'Root:\n
-    Error 1\n
+    Error 1 (code )\n
 Root.foo.bar:\n
-    Error 2\n
+    Error 2 (code )\n
 Root[baz]:\n
-    Error 3\n
+    Error 3 (code )\n
 foo.bar:\n
-    Error 4\n
+    Error 4 (code )\n
 [baz]:\n
-    Error 5\n
+    Error 5 (code )\n

2) Symfony\Component\Validator\Tests\ConstraintViolationTest::testToStringHandlesArrays
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
 'Root.property.path:
-    Array'
+    Array (code )'

3) Symfony\Component\Validator\Tests\ConstraintViolationTest::testToStringHandlesArrayRoots
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
 'Array.some_value:
-    42 cannot be used here'
+    42 cannot be used here (code )'

4) Symfony\Component\Validator\Tests\ConstraintViolationTest::testToStringOmitsEmptyCodes
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
 'Array.some_value:
-    42 cannot be used here'
+    42 cannot be used here (code )'

5) Symfony\Component\Validator\Tests\ConstraintViolationTest::testNonStringCode
Failed asserting that '42' is identical to 42.

6) Symfony\Component\Validator\Tests\Violation\ConstraintViolationBuilderTest::testNonStringCode
Failed asserting that '42' is identical to 42.
```

Commits
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22c58f5 [Validator] Fix tests