Treat reiser4 label "<none>" as meaning blank (#688760)

A reiser4 file system with a blank label is displayed as "<none>" by
debugfs.reiser4 and therefore GParted too.

    # mkfs.reiser4 -y -L "" -f /dev/sda14
    # debugfs.reiser4 /dev/sda14 2> /dev/null | grep label:
    label:		<none>

Additionally when displaying the Partition Information the following
warning is written to the terminal:

    (gpartedbin:19609): Gtk-WARNING **: Failed to set text from markup
    due to error parsing markup: Error on line 1 char 23: Element
    'markup' was closed, but the currently open element is 'none'

Fix by just assuming "<none>" always means blank.  If someone really
does set the label to "<none>" it won't be displayed.  This is equlivant
to how btrfs and nilfs2 are already handled.

Closes bug 688760 - GParted displays blank resier4 label as <none>