Fix for nil s3 input credentials

Array configuration values never return nil - if unspecified they return an empty array (at least from my testing). I noticed some people brought this issue up on google groups as well, so I went ahead and modified the credentials parameter to default to an empty array, and check for an empty array value - if empty it defaults to using environment variables. I left the original nil check in, though that could probably be removed as well (lines 71-73 of the file I'm merging in).

closes /elasticsearch/logstash/#1619