Failing S3 unit tests on master / cannot create folder on S3

[PVince81]

S3 unit tests fail on master 993376f
But they run fine on stable7 6995b96

(these are the commits before the regression I introduced here #11714)

Still, wondering why the tests fail on master when the files are identical.
I suspect it might be related to the upgrade to the new version.

Many tests fail with the same error and stack trace:

Aws\Common\Exception\InvalidArgumentException: You must specify a non-null value for the Body or SourceFile parameters.

/srv/www/htdocs/owncloud/apps/files_external/3rdparty/aws-sdk-php/Aws/Common/Client/UploadBodyListener.php:91
/srv/www/htdocs/owncloud/3rdparty/symfony/event-dispatcher/Symfony/Component/EventDispatcher/EventDispatcher.php:164
/srv/www/htdocs/owncloud/3rdparty/symfony/event-dispatcher/Symfony/Component/EventDispatcher/EventDispatcher.php:53
/srv/www/htdocs/owncloud/3rdparty/guzzle/common/Guzzle/Common/AbstractHasDispatcher.php:40
/srv/www/htdocs/owncloud/apps/files_external/3rdparty/aws-sdk-php/Guzzle/Service/Command/AbstractCommand.php:233
/srv/www/htdocs/owncloud/apps/files_external/3rdparty/aws-sdk-php/Guzzle/Service/Client.php:205
/srv/www/htdocs/owncloud/apps/files_external/3rdparty/aws-sdk-php/Guzzle/Service/Client.php:136
/srv/www/htdocs/owncloud/apps/files_external/3rdparty/aws-sdk-php/Guzzle/Service/Command/AbstractCommand.php:153
/srv/www/htdocs/owncloud/apps/files_external/3rdparty/aws-sdk-php/Guzzle/Service/Command/AbstractCommand.php:189
/srv/www/htdocs/owncloud/apps/files_external/3rdparty/aws-sdk-php/Guzzle/Service/Client.php:76
/srv/www/htdocs/owncloud/apps/files_external/3rdparty/aws-sdk-php/Aws/Common/Client/AbstractClient.php:108
/srv/www/htdocs/owncloud/apps/files_external/lib/amazons3.php:188
/srv/www/htdocs/owncloud/apps/files_external/lib/amazons3.php:188
/srv/www/htdocs/owncloud/tests/lib/files/storage/storage.php:521

It seems to be related to directory creation.
Do we need to pass a body when creating folders ?!

@butonic @helmutschneider any clue ?

[PVince81]

Note: we'll try and setup CI soon for external storage to catch such errors in the future (#11717)

Until now we need to run the unit tests locally...