Fix bug that fills up the logs

We have /var/log/stapled full with this strace log files

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/stapled/core/excepthandler.py", line 73, in stapled_except_handle
    yield  # do the "with stapled_except_handle(ctx):" code block
  File "/usr/lib/python3/dist-packages/stapled/core/stapleadder.py", line 166, in run
    self.add_staple(model)
  File "/usr/lib/python3/dist-packages/stapled/core/stapleadder.py", line 186, in add_staple
    for path, response in responses:
TypeError: 'NoneType' object is not iterable
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