Bug: get_content skips distribution_idx=0 due to falsy check in url_to_get_content

**Describe the bug**
In `src/aiod/calls/urls.py` line 61, the URL for `get_content` is built with:

url += f"/{distribution_idx}" if distribution_idx else ""

Since `distribution_idx=0` is the default value and `0` is falsy in Python,
the condition evaluates to `False` and `/0` is never appended to the URL.

This means `get_content(..., distribution_idx=0)` silently produces an
incorrect URL, making the first distribution unreachable by default.

**To Reproduce**
```python
from aiod.calls.urls import url_to_get_content

url = url_to_get_content("datasets", "abc123", distribution_idx=0)
print(url) # prints: .../datasets/abc123/content ← missing /0
```

**Expected behavior**
`.../datasets/abc123/content/0`

**Actual behavior**
`.../datasets/abc123/content`

**Proposed Fix**
Change line 61 in `src/aiod/calls/urls.py` from:
```python
url += f"/{distribution_idx}" if distribution_idx else ""
```
to:
```python
url += f"/{distribution_idx}" if distribution_idx is not None else ""
```

**Environment**
- aiondemand version: 0.2.5
- Python version: 3.x

I am solving this bug.