с указанием регистра:
CASE
WHEN ((@Latitude IS NOT NULL) AND (@Longitude IS NOT NULL))
THEN geography::Point(@Latitude, @Longitude, 4326)
ELSE NULL
END
или вариант с if:
DECLARE @Location geography = NULL
IF (@Latitude IS NOT NULL AND @Longitude IS NOT NULL)
SET @Location = geography::Point(@Latitude, @Longitude, 4326);