Improve prefix_sum.py (TheAlgorithms#12560)

* Update prefix_sum.py

Index Validation for get_sum

Raises ValueError if start or end is out of range or start > end.
Handles cases where the array is empty.
✅ Empty Array Support

If an empty array is passed, get_sum raises an appropriate error instead of failing unexpectedly.
✅ Optimized contains_sum Initialization

Initializes sums with {0} for efficient subarray sum checking.

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* Update prefix_sum.py

* Update prefix_sum.py

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Update prefix_sum.py

* Update prefix_sum.py

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Maxim Smolskiy <mithridatus@mail.ru>

data_structures/arrays/prefix_sum.py

@@ -30,11 +30,29 @@ def get_sum(self, start: int, end: int) -> int:
         5
         >>> PrefixSum([1,2,3]).get_sum(2, 2)
         3
+        >>> PrefixSum([]).get_sum(0, 0)
+        Traceback (most recent call last):
+        ...
+        ValueError: The array is empty.
+        >>> PrefixSum([1,2,3]).get_sum(-1, 2)
+        Traceback (most recent call last):
+        ...
+        ValueError: Invalid range specified.
         >>> PrefixSum([1,2,3]).get_sum(2, 3)
         Traceback (most recent call last):
         ...
-        IndexError: list index out of range
+        ValueError: Invalid range specified.
+        >>> PrefixSum([1,2,3]).get_sum(2, 1)
+        Traceback (most recent call last):
+        ...
+        ValueError: Invalid range specified.
         """
+        if not self.prefix_sum:
+            raise ValueError("The array is empty.")
+
+        if start < 0 or end >= len(self.prefix_sum) or start > end:
+            raise ValueError("Invalid range specified.")
+
         if start == 0:
             return self.prefix_sum[end]