Handle Direct Edits with no xpath

Reproduction

Since the v4 update of Drive, some files downloaded with direct edit are not correctly updated. The error can be reproduced by following these steps:

• Make a direct edit on a file with v3.1.2,
• Close the Drive client,
• Make a change to the doc,
• Update Drive to v4.0.2.

Result:

The document won't be updated on the platform and the logs will show the error returned by the server on the Blob.Attach operation (see related SUPNXP).

Details:

The direct edit has been enabled on all blobs, not only the main blob of a document. To handle this, we set an extended attribute on the downloaded file that includes the xpath of the blob in the document.
Problem is that this xpath is set upon downloading the file. If the file was downloaded by Drive 3.1.2, this xpath isn't set. When sending changes, Drive 4.0.2 tries to attach the blob to a non-existant xpath.
Files which were downloaded with direct edit in Drive 4+ shouldn't be affected.
The solution is to add a default xpath on files when uploading changes.