handle Optional and Required constraints from XML or YAML sources correctly
merged branch xabbuh/issue7575 (PR #7930)

This PR was submitted for the 2.1 branch but it was merged into the 2.2 branch instead (closes #7930).

Discussion
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handle Optional and Required constraints from XML or YAML sources correc...

...tly

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7575
| License       | MIT
| Doc PR        | symfony/symfony-docs#2432

Commits
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a93a23a handle Optional and Required constraints from XML or YAML sources correctly