save: add empty metadata if reading fails

save: add empty metadata if reading fails

If we cannot read a file's metadata, we must add an entry in the
repository nonetheless because the bupm contents must match the
tree contents.

Add a dummy (empty) metadata entry in this case, as is already
done when reading a directory's metadata.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
(cherry picked from commit 47891d8951a95b8e0d9ca94387107cdf12ca3d3c)
Tested-by: Rob Browning <rlb@defaultvalue.org>