mkfs.f2fs: show the info of volume label only when specify it

mkfs.f2fs: show the info of volume label only when specify it

Currently, when format disk used with mkfs.f2fs, even if the volume label
is not specified, it still shows the info of volume label with no content
due to the check condition is always true, this patch fixes it.

[root@localhost home]# mkfs.f2fs -f /dev/sdb1

	F2FS-tools: mkfs.f2fs Ver: 1.12.0 (2018-10-08)

Info: Disable heap-based policy
Info: Debug level = 0
Info: Label =
Info: Trim is enabled

Signed-off-by: Tiezhu Yang <kernelpatch@126.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>