Fixing web crashes in pending and dashboard when clean.sh script is called

Hope you will not again refuse that kind of patch, it's used to prevent web gui crashed when the clean.sh script is called and web is still running.

case 1:
 File "/Users/xxx/Project/cuckoo-dma/web/analysis/views.py", line 63, in pending
    for task in tasks:
TypeError: 'NoneType' object is not iterable
  

case 2:
  tasks += db.list_tasks(offset=offset, status=TASK_REPORTED)
TypeError: unsupported operand type(s) for +=: 'NoneType' and 'NoneType'