i-t: don't try to import from empty cache

Chases 7c64ee9e7731b7ad39e300b4a422892dbe8d4b23
("zfs-import-{cache,scan}: change condition to FileNotEmpty")

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #12108