[Validator] Fixed Constraint class to not destroy custom groups upon serialization
bug #11691 [Validator] Fixed Constraint class to not destroy custom groups upon serialization (webmozart)

This PR was merged into the 2.6-dev branch.

Discussion
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[Validator] Fixed Constraint class to not destroy custom groups upon serialization

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11638
| License       | MIT
| Doc PR        | -

Commits
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d405f43 [Validator] Fixed Constraint class to not destroy custom groups upon serialization