libblkid: cleanup nonnull attribute usage

libblkid: cleanup nonnull attribute usage

 - don't mix __attribute__((nonnull)) and "if (!argument) return" for
   the same function. The "if ()" check will be *silently* optimized
   out by gcc.

 - use __attribute__((nonnull)) for functions where is impossible to
   return error code. For example "is" and "has" functions or functions
   where we return unsigned types and zero is a valid value.

Signed-off-by: Karel Zak <kzak@redhat.com>