Reject null BitonicSort input explicitly (#7550)

Update BitonicSort.java

Added a descriptive null-input validation and a focused regression test.

Author: cragkhit

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

@@ -21,6 +21,9 @@ private enum Direction {
      */
     @Override
     public <T extends Comparable<T>> T[] sort(T[] array) {
+        if (array == null) {
+            throw new IllegalArgumentException("The input array cannot be null");
+        }
         if (array.length == 0) {
             return array;
         }

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

@@ -1,8 +1,17 @@
 package com.thealgorithms.sorts;
 
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
 public class BitonicSortTest extends SortingAlgorithmTest {
     @Override
     SortAlgorithm getSortAlgorithm() {
         return new BitonicSort();
     }
+
+    @Test
+    void shouldRejectNullArray() {
+        assertThrows(IllegalArgumentException.class, () -> getSortAlgorithm().sort((Integer[]) null));
+    }
 }