Return None if the filesystem has no label (#1038590)
Return None if the filesystem has no label (#1038590)

Related: rhbz#1038590

If the filesystem has no label the regular expression that matches the
output will match an empty string. Change that to None, because that's
what it means.

Signed-off-by: mulhern <7b51bcf507bcd7afb72bf8663752c0ddbeb517f6@redhat.com>

vpodzime/blivet,vojtechtrefny/blivet,rhinstaller/blivet,vojtechtrefny/blivet,rvykydal/blivet,dwlehman/blivet,jkonecny12/blivet,rvykydal/blivet,AdamWill/blivet,AdamWill/blivet,dwlehman/blivet,vpodzime/blivet,rhinstaller/blivet,jkonecny12/blivet