Update aws_import_arbiter.py

Added filtering for NoneType, to avoid the following
[1374137157] Error :   Instance AWS raised an exception sequence item 0: expected string, NoneType found. Log and continue to run
[1374137157] Error :   Back trace of this remove: Traceback (most recent call last):
  File "/opt/shinken/local/lib/python2.7/site-packages/shinken/daemons/arbiterdaemon.py", line 285, in load_config_file
    r = inst.get_objects()
  File "/opt/shinken/local/lib/python2.7/site-packages/shinken/modules/aws_import_arbiter.py", line 126, in get_objects
    h[prop] = ','.join(v)
TypeError: sequence item 0: expected string, NoneType found