refactor: fix phpstan lvl5 error for AwsS3 adapter #26

Job	Run time
bc-break / BC Check	18s
test / Build (8.0)	16s
linter / PHPStan (8.0)	37s
test / Build (8.1)	13s
linter / PHP-CS-Fixer	31s
linter / PHPStan (8.1)	34s
test / Adapter (8.2, async-aws/simple-s3:^1.0, spec/Gaufrette/Adapter/AsyncAwsS3Spec.php)	13s
test / Build (8.2)	21s
linter / PHPStan (8.2)	44s
test / Adapter (8.2, aws/aws-sdk-php:^3.158, spec/Gaufrette/Adapter/AwsS3Spec.php)	21s
test / Adapter (8.2, google/apiclient:^2.12, spec/Gaufrette/Adapter/GoogleCloudStorageSpec.php)	27s
test / Adapter (8.2, doctrine/dbal:^2.3, spec/Gaufrette/Adapter/DoctrineDbalSpec.php)	15s
test / Adapter (8.2, doctrine/dbal:^3.4, spec/Gaufrette/Adapter/DoctrineDbalSpec.php)	19s
test / Adapter (8.2, league/flysystem:^1.0, spec/Gaufrette/Adapter/FlysystemSpec.php)	20s
test / Adapter (8.2, microsoft/azure-storage-blob:^1.0, spec/Gaufrette/Adapter/AzureBlobStore)	15s
test / Adapter (8.2, mongodb/mongodb:^1.1, spec/Gaufrette/Adapter/GridFSSpec.php)	20s
test / Adapter (8.2, phpseclib/phpseclib:^2.0, spec/Gaufrette/Adapter/PhpseclibSftpSpec.php)	19s
	6m 23s

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