Fix issues with 0 sized info files for raw backup

When backuping from devices that have configured to use raw backup and
that might disconnect from the network (ie. laptops) you end up once in
a while with 0 size info file (and backup file).
btrbk don't know how to handle 0 file and stop backing up until the zero
size file is removed.
With this change 0 size info file will be ignored, and hence the backup
for the given backup will be redone.

Signed-off-by: Matthieu Patou <mat@matws.net>

btrbk

@@ -2005,6 +2005,7 @@ sub system_read_raw_info_dir($)
     cmd => [ 'find', { unsafe => $droot->{PATH} },
              '-maxdepth', '1',
              '-type', 'f',
+             '! -size', 0,
              '-name', '\*.btrfs.\*info',  # match ".btrfs[.gz|bz2|xz][.gpg].info"
              '-exec', 'echo INFO_FILE=\{\} \;',
              '-exec', 'cat \{\} \;'