Be sure that the discovered filesystem is not null.

Be sure that the discovered filesystem is not null.

- While scanning for a filesystem on a disk, the klibc utility can
  return a null string if it not finds a filesystem (e.g. on a container
  partition), and grepping for a null string either reports true if
  escaped or hangs grep if not escaped. Both conditions must be avoided
  in order to report that the kernel supports the filesystem named as
  the string.

Signed-off-by: Marco Amadori <marco.amadori@gmail.com>