===== 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 calling `statistics.median`.

```python
import statistics

def robust_center(values):
    """
    Returns the median of a list of numbers, or None if the list is empty.
    """
    if not values:
        return None
    return statistics.median(values)

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

### Explanation:
*   **`if not values:`**: In Python, an empty list is "falsy." This check efficiently catches empty lists and returns `None` immediately, preventing the `statistics.median` function from raising a `StatisticsError`.
*   **`statistics.median`**: This function handles both odd and even-lengthed lists automatically (for even lists, it returns the average of the two middle values).