erofs: allow empty device tags in flatdev mode

Device tags aren't actually required in flatdev mode, thus fix mount
failure due to empty device tags in flatdev mode.

Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
Fixes: 8b465fec ("erofs: support flattened block device for multi-blob images")
Reviewed-by: Jia Zhu <zhujia.zj@bytedance.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230915082728.56588-1-jefflexu@linux.alibaba.com


Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>