MixedCut.truncate: fix the case when only PaddingCuts are left

[flyingleafe]

I encountered another couple of bugs after simulating long conversations with ConversationalMeetingSimulator, cutting them into windows and loading the windows' audios:

• If a window happened to be inside an offset region without any cuts, a PaddingCut was returned for which the audio (zeros array) could not be loaded, since num_samples property was not defined;
• If it happened so that only PaddingCuts remained after truncation, this case was not properly handled (in the same way as "no cuts remained" case), which resulted in the IndexError exception at the last lines of truncate method (where we attempt to set SNR of first non-padding track to None)

This PR fixes those.

[desh2608]

Thanks for catching this edge case. Can you also add a test which would have failed before making this change but passes now?

[flyingleafe]

@desh2608 Sorry for such a delay, added the test.
The case with IndexError seems to be quite exotic - it replicates only when two or more PaddingCuts remained after truncation. Yet, apparently, cutting the output of ConversationalMeetingSimulator can lead to such cases.

[desh2608]

Thanks. LGTM!