===== ISTEM =====
Using the `median` function from Python's standard `statistics` module, write `robust_center(values)` that returns the median of a list of numbers, or None for an empty list.

===== HAM YANIT =====
Here's a Python function `robust_center(values)` that uses the `median` function from the `statistics` module and returns `None` for an empty list:

```python
import statistics

def robust_center(values):
    if not values:
        return None
    return statistics.median(values)
```

### Explanation:
- The function first checks if the input list `values` is empty using `if not values`.
- If it's empty, it returns `None`.
- Otherwise, it uses `statistics.median()` to compute the median of the list and returns it.

### Example usage:

```python
print(robust_center([1, 2, 3]))      # Output: 2
print(robust_center([1, 3]))         # Output: 2.0
print(robust_center([]))            # Output: None
```

This function is robust in the sense that it handles empty lists gracefully and returns the median as expected.