Don't rename first blob of cached conversions when multiple blobs are cached

The first blob is renamed in the conversion service but it should not when the cached entry is a list of blobs.

// we need to reset the filename if result came from cache because it's just a hash
result.getBlob().setFilename(null);

current behavior:

the first time the converter returns a bloblist:

• file1.jpeg
• file2.jpeg

second time the cache returns:

• nxblob*
• file2.jepg

Expected behavior: cached blobs heep their original filename

PR: https://github.com/nuxeo/nuxeo/pull/1906