===== 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 returns the median of a list of numbers using the `statistics.median` function, and returns `None` if the list is empty:

```python
import statistics

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

### Example Usage:

```python
print(robust_center([1, 3, 5]))       # Output: 3
print(robust_center([1, 2, 3, 4]))   # Output: 2.5
print(robust_center([]))            # Output: None
```

### Notes:
- The `statistics.median` function handles both even and odd length lists correctly.
- If the list is empty, the function returns `None` as requested.