Remove 'node has no file' error

[MrCreosote]

With the blobstore, it's impossible for a node to be file-less.
See https://codecov.io/gh/kbaseapps/DataFileUtil/pull/72/changes

[MrCreosote]

Note that the blob store does accept zero-length files - add a test for this.

[MrCreosote]

Note that the blob store does accept zero-length files

Does it? Double check this.