===== 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 the `statistics.median` function while adding a check to handle the empty list case, as `statistics.median` raises a `StatisticsError` if the input sequence is empty.

```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 of usage:
print(robust_center([1, 3, 5]))      # Output: 3
print(robust_center([1, 3, 5, 7]))   # Output: 4.0
print(robust_center([]))             # Output: None
```

### Explanation:
1.  **`if not values:`**: This checks if the list is empty. In Python, an empty list evaluates to `False` in a boolean context.
2.  **`return None`**: If the list is empty, the function exits early, returning `None` as requested.
3.  **`statistics.median(values)`**: If the list contains elements, the standard library function calculates the median (the average of the two middle numbers for even-length lists, or the middle number for odd-length lists).