api_tsuite, not fail if ID params does not exist for Testlink webapp

[netzulo]

api_tsuite don't handle if ID does not exist for testlink, maybe error it's at error response structure

>>> tsuite = tlm.api_tsuite(1)
2018-03-16 12:07:26,206 - qatestlink - INFO - POST request:
2018-03-16 12:07:26,486 - qatestlink - INFO - POST response:
2018-03-16 12:07:26,486 - qatestlink - INFO -     status_code=200
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "qatestlink/core/testlink_manager.py", line 356, in api_tsuite
    properties = res_param.get('struct')['member']
TypeError: 'NoneType' object has no attribute '__getitem__'
>>> tsuite = tlm.api_tsuite(2)
2018-03-16 12:07:29,425 - qatestlink - INFO - POST request:
2018-03-16 12:07:29,548 - qatestlink - INFO - POST response:
2018-03-16 12:07:29,548 - qatestlink - INFO -     status_code=200
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "qatestlink/core/testlink_manager.py", line 356, in api_tsuite
    properties = res_param.get('struct')['member']
TypeError: 'NoneType' object has no attribute '__getitem__'
>>> tsuite = tlm.api_tsuite(3)
2018-03-16 12:07:31,526 - qatestlink - INFO - POST request:
2018-03-16 12:07:31,653 - qatestlink - INFO - POST response:
2018-03-16 12:07:31,653 - qatestlink - INFO -     status_code=200
>>>