[CrashManager] No signature suggestion causes 500 error

Creating a bucket from a "No crash detected" CrashEntry results in a 500 server error.

Traceback (most recent call last):
  File "/srv/venv/lib/python3.8/site-packages/django/core/handlers/base.py", line 113, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/srv/FuzzManager/server/crashmanager/views.py", line 396, in newSignature
    'proposedSig': json.loads(proposedSignature),
  File "/usr/lib/python3.8/json/__init__.py", line 357, in loads
    return _default_decoder.decode(s)
  File "/usr/lib/python3.8/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib/python3.8/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)

We should handle this case and show the new signature form with no suggestion.

Start in crashmanager/views.py at newSignature, especially the proposedSignature handling shown in the traceback. Reproduce bucket creation from a "No crash detected" CrashEntry, then verify the request no longer returns a 500 and instead displays the new signature form without a suggestion.