===== 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 is the `robust_center` function that calculates the median of a list of numbers using Python's standard `statistics` module, or returns None if the list is empty:

```python
import statistics

def robust_center(values):
    if len(values) == 0:
        return None
    else:
        return statistics.median(values)
```

You can use this function like this:

```python
values = [1, 2, 3, 4, 5]
print(robust_center(values))  # Output: 3

values = []
print(robust_center(values))  # Output: None
```

This function first checks if the input list `values` is empty. If it is, the function returns `None`. Otherwise, it calculates and returns the median of the list using the `median` function from Python's `statistics` module.