Ignore empty bucket keys in S3 import

Erwan Rouchet
2022-06-28T10:19:40Z

Refs requests#709

Bucket keys can be completely empty, or have just a /, or have an empty "folder" (since there is no concept of folder in S3) : a//////////b. Empty keys cause the task to try to create folders with empty names, which is not allowed by Arkindex. Those keys should just be ignored when building the graph.