===== 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 =====
To implement this, you can use a simple `if` check to handle the empty list case before passing the data to the `statistics.median` function.

```python
import statistics

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

# Examples:
# print(robust_center([1, 3, 3, 6, 7, 8, 9])) -> 6
# print(robust_center([]))                   -> None
```

### Explanation:
1.  **`import statistics`**: This imports Python's built-in module for mathematical statistics.
2.  **`if not values:`**: This evaluates to `True` if the list is empty, allowing us to return `None` explicitly and avoid a `StatisticsError` which `statistics.median` would otherwise raise on an empty sequence.
3.  **`statistics.median(values)`**: This calculates the median:
    *   If the list has an odd number of elements, it returns the middle value.
    *   If the list has an even number of elements, it returns the average of the two middle values.