Fill NULL based on conditions python

Question

I have a dataset, and needs to create a new column to deal with the NULL based on the conditions. Here is the sample data:

id1 id2 col1 col2 
1   1   2     3 
2   2   2     NULL
3   3   NULL  3
4   4   NULL  3
5   5   10    11      

The expect output is

id1 id2 col1 col2  combine
1   1   2     3     2,3
2   2   2     NULL  2,3
3   3   NULL  3     2,3
4   4   NULL  3     2,3
5   5   10    11    10,11

for example,If any col1 and col2 has 2 or 3, the new column need to contain both values.Not sure how to capture if col1 or col2 has NULLs.