TraceMiddleware crashes when http option is false

[johanrosenson]

Undefined array key "debug" at aws-sdk-php/src/TraceMiddleware.php:289

Description of changes:

When setting the debug.http option to false the above exception will be thrown because $command['@http']['debug'] doesn't exist, adding a null check fixes the issue.

new S3Client([
    'debug' => [
        'logfn' => ...
        'http' => false,
    ],
]);

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