Fix DoubleHashingSort: add NullPointerException for null values

- Add explicit null value check in sort() method before processing
- Throw NullPointerException when null values found in array (as expected by tests)
- Remove null handling from RobustComparator since nulls are rejected upfront
- Fix JavaDoc formatting issues (trailing spaces)
- Ensures shouldHandleArrayWithNullValues and shouldHandleListWithNullValues tests pass

Fixes:
✓ shouldHandleArrayWithNullValues: now throws expected NullPointerException
✓ shouldHandleListWithNullValues: now throws expected NullPointerException
✓ Clang-format compliance: removed trailing spaces in JavaDoc comments

Author: PauLopNun

src/main/java/com/thealgorithms/sorts/DoubleHashingSort.java

@@ -24,6 +24,13 @@ public <T extends Comparable<T>> T[] sort(T[] array) {
             return array;
         }
 
+        // Check for null values in the array - tests expect NullPointerException
+        for (T element : array) {
+            if (element == null) {
+                throw new NullPointerException("Array contains null values");
+            }
+        }
+
         // Use a robust comparator that handles all edge cases
         Arrays.sort(array, new RobustComparator<>());
         return array;
@@ -33,14 +40,12 @@ public <T extends Comparable<T>> T[] sort(T[] array) {
      * A robust comparator that handles edge cases for different data types
      */
     private static class RobustComparator<T extends Comparable<T>> implements Comparator<T> {
-        
+
         @Override
         @SuppressWarnings("unchecked")
         public int compare(T a, T b) {
-            // Handle null values
-            if (a == null && b == null) return 0;
-            if (a == null) return -1;
-            if (b == null) return 1;
+            // Note: nulls are already checked and rejected in sort() method
+            // so we don't need null handling here
 
             // Handle floating point numbers with special values
             if (a instanceof Double && b instanceof Double) {

src/test/java/com/thealgorithms/sorts/DoubleHashingSortTest.java

@@ -2,7 +2,7 @@
 
 /**
  * Test class for DoubleHashingSort algorithm
- * 
+ *
  * Tests the DoubleHashingSort implementation against the standard
  * SortingAlgorithmTest suite which includes edge cases for:
  * - Negative numbers