Fix stalin_sort raising IndexError on an empty list

The function read sequence[0] unconditionally. Return an empty list for empty
input and add doctests for the empty and single-element cases.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: Darkslayer3324j

sorts/stalin_sort.py

@@ -32,7 +32,16 @@ def stalin_sort(sequence: list[int]) -> list[int]:
 
     >>> stalin_sort([1, 2, 8, 7, 6])
     [1, 2, 8]
+
+    >>> stalin_sort([])
+    []
+
+    >>> stalin_sort([7])
+    [7]
     """
+    if not sequence:
+        return []
+
     result = [sequence[0]]
     for element in sequence[1:]:
         if element >= result[-1]: