[Backport #2020 to 5.x] fix: remove redundant is_array() check in Serializer

[github-actions]

Automated backport of #2020 to branch 5.x.

Do not modify this pull request.

Summary

• PHPStan reported is_array() on $declaration always evaluates to true in Serializer::doDeserializeProperty()
• Root cause: after checking is_string($declaration) with a separate if, the subsequent is_array() checks were redundant since the type union is string|array<string>
• Restructured to use if/elseif branching so the string case is handled first and the array branches follow naturally without needing explicit is_array() guards

Test plan

• composer analyse — 0 errors
• composer test — 1037 tests pass
• CS Fixer clean
• Rector clean

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