Panic on empty volume name about ntfs

[ColinFinck]

Thanks a lot for your report and the additional comment in #10 (comment)!
I finally realized that these were actually two bugs:

• NtfsVolumeName must support empty strings as volume names. I previously only encountered filesystems that removed the entire $VOLUME_NAME attribute when the volume name was cleared. But apparently, this is not guaranteed.
• data_position must also return a position when stream_position equals the value length. This happens when we seek to the very end of a value, but also when the value is zero-sized.

Both bugs have been fixed in 61d13c6 and a063e4a.

Note that $VOLUME_NAME is always a resident attribute. The Data Runs concept only applies to non-resident attributes, hence this bug needs to be clearly separated from #10.

[leofidus]

I can confirm that it works now. After your commits I get an empty string as volume name, as expected. Thanks.