Combining nullif while checking if field is an integer with a case statement
I believe Azure supports TRY_CAST.  


  (Returns a value cast to the specified data type if the cast succeeds;
  otherwise, returns null.)


Here are three examples that seem to demonstrate what you're after:

--Column is blank, then NULL
DECLARE @YourCol1 VARCHAR(10) = ' '

SELECT CASE 
        WHEN @YourCol1 = ' '
            THEN NULL
        ELSE ISNULL(TRY_CAST(@YourCol1 AS TINYINT), 255)
        END AS YourCol1

--Column won't successfully CAST to TinyInt, then 255
DECLARE @YourCol2 VARCHAR(10) = 'abc'

SELECT CASE 
        WHEN @YourCol2 = ' '
            THEN NULL
        ELSE ISNULL(TRY_CAST(@YourCol2 AS TINYINT), 255)
        END AS YourCol2

--Column successfully CASTS to TinyInt, then actual value
DECLARE @YourCol3 VARCHAR(10) = '21'

SELECT CASE 
        WHEN @YourCol3 = ' '
            THEN NULL
        ELSE ISNULL(TRY_CAST(@YourCol3 AS TINYINT), 255)
        END AS YourCol3