btrbk: ignore zero-size info files

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 \{\} \;'