fix(metadata): drop unreachable array guards on serializer attributes

[soyuka]

One of three PRs finishing the 4.3 CI unblock started in #8458. Pure dead-code removal, no behaviour change.

PHPStan (PHP 8.5) fails on 4.3 with:

src/Metadata/Property/Factory/SerializerPropertyMetadataFactory.php
  61  Call to function is_array() with non-empty-array will always evaluate to true.
  65  Call to function is_array() with non-empty-array will always evaluate to true.

getEffectiveSerializerAttributes() (:239-278) is declared @return (array|null)[] and every return path yields array|null, so the !\is_array() branches could never execute. Introduced by copy-paste in 64b46b2d0 (#7629), where attributes support was cloned from the groups block above.

Why the sibling guards are left alone

The structurally identical guards for $normalizationGroups / $denormalizationGroups a few lines up do not error, and that asymmetry is correct rather than an oversight:

• getEffectiveSerializerGroups() returns (string[]|string|null)[] — the string member keeps !is_array() reachable.
• It is grounded in Symfony's contract, not just our docblocks. AbstractNormalizer::getGroups() does \is_scalar($groups) ? (array) $groups : $groups, explicitly supporting a scalar groups. AbstractNormalizer::isAllowedAttribute() has no equivalent coercion for ATTRIBUTES; a non-array simply falls through to return true.

So attributes is array-only by Symfony's own contract. Coercing a scalar here would make this factory diverge from the serializer that consumes the value.