cppcheck: zpool_main.c possible null pointer dereference

Explicitly check for NULL to satisfy cppcheck that "val" can never
be NULL when passed to printf(). This looks like a false positive
since is_blank_str() can never take the false conditional branch
when passed a NULL. But there's no harm in adding the extra check.

Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11508