vad_infer.py script fails with `IndexError` if option `--dont_auto_split` is not set
**Describe the bug**

`NeMo/examples/asr/vad_infer.py` script fails with `IndexError` if option `--dont_auto_split` is not set and at least 1 `.wav` file is long enough so that `nemo.collections.asr.parts.utils.vad_utils.prepare_manifest()` function split the `.wav` file.

**Steps/Code to reproduce bug**

```
mkdir -p ~/debug_data
wget http://i13pc106.ira.uka.de/~jniehues/IWSLT-SLT/data/eval/en-de/IWSLT-SLT.tst2019.en-de.tgz -O ~/debug_data/IWSLT-SLT.tst2019.en-de.tgz
tar xzf ~/debug_data/IWSLT-SLT.tst2019.en-de.tgz -C ~/debug_data/
cd ~/NeMo/examples/asr/
wget https://raw.githubusercontent.com/NVIDIA/NeMo/feat/asr/iwslt_audio_to_nemo_format/scripts/dataset_processing/prepare_iwslt_audio_data.py
python prepare_iwslt_audio_data.py -a ~/debug_data/IWSLT.tst2019/wavs/ -t ~/debug_data/IWSLT.tst2019/IWSLT.TED.tst2019.en-de.en.xml -o ~/debug_data/IWSLT.tst2019/manifest.json
python vad_infer.py --dataset ~/debug_data/IWSLT.tst2019/manifest.json --out_dir ~/debug_data/IWSLT.tst2019/vad --vad_model vad_marblenet
```


**Expected behavior**

No errors

**Environment overview (please complete the following information)**

 - Environment location: Bare-metal
 - Method of NeMo install: pip install nemo_toolkit[all]

**Environment details**

If NVIDIA docker image is used you don't need to specify these.
Otherwise, please provide:
- OS version: Ubuntu 20.04.2 LTS
- PyTorch version: 1.8.1
- Python version: 3.8.10

