EmpNr | datefrom   | dateto     | hours
 0815 | 01.01.2019 | 03.01.2019 | 15
 0815 | 05.01.2019 | 15.01.2019 | 15
 0815 | 20.01.2019 | 31.12.9999 | 40

0815 | 01.01.2019 | 15
0815 | 02.01.2019 | 15
0815 | 03.01.2019 | 15
0815 | 04.01.2019 | NULL
0815 | 05.01.2019 | 15
...
0815 | 15.01.2019 | 15
0815 | 16.01.2019 | NULL
0815 | 17.01.2019 | NULL
0815 | 18.01.2019 | NULL
0815 | 19.01.2019 | NULL
0815 | 20.01.2019 | 40
0815 | 21.01.2019 | 40
...
0815 | 31.01.2019 | 40

declare @year int = 2019, @month int = 1;
WITH numbers
as
(
    Select 1 as value
    UNion ALL
    Select value + 1 from numbers
    where value + 1 <= Day(EOMONTH(datefromparts(@year,@month,1)))
)
SELECT b.empnr, b.hours, datefromparts(@year,@month,numbers.value) Datum FROM numbers left outer join
emptbl b  on b.empnr = '0815' and  (datefromparts(@year,@month,numbers.value) >=  b.datefrom and datefromparts(@year,@month,numbers.value) <= case b.dateto )