title: Loop through two lists if list maybe None or size of two list maybe different
body: <p>what is the best way for me to iterate through two lists like following. the only things is two lists maybe None or size is different
  I cannot use zip because list maybe None or size maybe different, I cannot use concatenation neither because one of list maybe None</p>
  
  <pre><code>iter1 = [1,2,3,4]
  iter2 = ['a', 'b', 'c', 'd', 'e']
  for a in (iter1+iter2):
      print(a)
  ============
  output
  1
  2
  3
  4
  a
  b
  c
  d
  e
  
  iter1 = None
  iter2 = ['a', 'b', 'c', 'd', 'e']
  a
  b
  c
  d
  e
  
  iter1 = None
  iter2 = None
  nothing print
  </code></pre>