Transcription failes when there is no audio

```
INFO:faster_whisper:Processing audio with duration 00:25.963
INFO:faster_whisper:VAD filter removed 00:25.963 of audio
INFO:faster_whisper:Detected language 'en' with probability 0.52
WARNING:root:Worker failed with exception
Traceback (most recent call last):
File "/nix/store/rmz3azc2p4vh87is42qfjkzimmi1nj48-transcribee-worker-env/lib/python3.11/site-packages/transcribee_worker/whisper_transcribe.py", line 34, in move_space_to_prev_tok
en
last_paragraph = next(iter)
^^^^^^^^^^
StopIteration

The above exception was the direct cause of the following exception:
```