Grouping by date

This is my data set,  how to  grouping by next occurance of Null

Start_Date      End_Date

2020-01-15     NULL
2020-01-15     2020-01-15
2020-01-17     2020-01-17
2020-01-21     2020-01-17
2020-01-22     2020-01-21
2020-01-23     2020-01-22
2020-01-24     2020-01-23
2020-01-24     2020-01-24 ---> This End_Date will be End_Date for the Start_Date = 2020-01-15
2020-12-28      NULL
2020-12-28     2020-12-28
2020-12-28     2020-12-28
2020-12-28     2020-12-28
2020-12-28     2020-12-28
2020-12-28     2020-12-28
2020-12-28      2020-12-28
2021-01-12      2021-01-11---> This End_Date will be End_Date for the Start_Date =2020-12-28
2021-09-22      NULL
2021-09-22      2021-09-22
2021-09-29      2021-09-22
2021-09-29      2021-09-29
2021-11-12         2021-11-12 ---> This End_Date will be End_Date for the Start_Date =2021-09-22

Final Result result will be like this

2020-01-15       2020-01-24
2020-12-28       2021-01-11
2021-09-22       2021-11-12