Fix error message when zfs module are already unloaded

Using zfs-sh -u on linux will fail with inaccurate message when the
zfs modules are already unloaded. Deal with the case where a module
is already unloaded; its USE_COUNT will be the empty string

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Adam Moss <c@yotes.com>
Closes #11627