How can I back fill null values in bigquery?

I'm trying to perform a null backfill, similar to Panda's dataframe bfill, in BigQuery. Reading the docs, the last_value function seems to be a good choice. However, this leaves some null spots until it finds the first value (quite reasonable, given the name of the function). How can I backfill those null? Or I just have to drop them?

This is a sample query:

select table_path.*, last_value(sn_6 ignore nulls) over (order by time)
from (select 1 as time, null as sn_6 union all
      select 2, 1 union all
      select 3, null union all
      select 4, null union all
      select 5, null union all
      select 6, 0 union all
      select 7, null union all
      select 8, null
     ) table_path;

Actual output:

time    sn_6    f0_
1       null   null
2         1     1
3       null    1
4       null    1
5       null    1
6         0     0
7       null    0
8       null    0

Desired output:

time    sn_6    f0_
1       null    1 <---Back fill all the gaps!
2         1     1
3       null    1
4       null    1
5       null    1
6         0     0
7       null    0
8       null    0

The real data has a timestamp column followed by 6 float columns and there are null values everywhere.