Fix ZeroDivisionError in DistributedBucketSampler
In the `_create_buckets` method of the `DistributedBucketSampler` class, a ZeroDivisionError was being raised when `len_bucket` was zero. This fix adds a conditional check to skip the problematic division if `len_bucket` is zero, thus avoiding the error and ensuring that the rest of the code functions as expected.

# What does this PR do ?

Fixes a ZeroDivisionError in the DistributedBucketSampler class to ensure proper functionality.

**Collection**: TTS Collection

# Changelog 
- Added a conditional check in the `_create_buckets` method to prevent division by zero when `len_bucket` is zero.

# Usage

```python
from nemo.collections.tts.data.dataset import DistributedBucketSampler

# Initialize the sampler with your dataset, batch_size, and boundaries
sampler = DistributedBucketSampler(dataset, batch_size, boundaries)

# Use the sampler in your training loop
for batch in sampler:
    # Process the batch
```

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