list2 += [None for i in range(len(list1)-len(list2))]

In [194]: l1 = range(5)
 
In [195]: l2 = range(2)
 
In [196]: l2.extend(["null"]*(len(l1)-len(l2)))
 
In [197]: l2
Out[197]: [0, 1, 'null', 'null', 'null']
 
# 如果兩個list長度相差很多，可以用iterator避免構造臨時的list
In [198]: l2 = range(2)
 
In [199]: l2.extend("null" for _ in range(len(l1)-len(l2)))
 
In [200]: l2
Out[200]: [0, 1, 'null', 'null', 'null']