Counting null values as unique value

select  count(distinct col1) + count(distinct case when col1 is null then 1 end)
from    YourTable

select 
   count(0) 
from
  (
      select distinct hour from hours
  )

SqlFiddle

SELECT
      ( SELECT COUNT(DISTINCT hour)
        FROM hours
      )
    + CASE WHEN EXISTS
           ( SELECT *
             FROM hours
             WHERE hour IS NULL
           )
        THEN 1 
        ELSE 0
      END
   AS result
FROM dual ;

if hour is a number, then if it can only be an integer:

select count(distinct coalesce(hour, 0.1)) cnt from test;

otherwise if it can be any floating point, change NULL to a char string.

eg

select count(distinct coalesce(to_char(hour), 'a')) cnt from test;