fix: resolve checkstyle, dead code, and add test coverage

Author: anshullakra007

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

@@ -24,6 +24,9 @@
  */
 public class ConcurrentMergeSort {
 
+    private ConcurrentMergeSort() {
+    }
+
     /**
      * Fallback threshold where the algorithm switches to standard sequential
      * Merge Sort to prevent thread-creation overhead from ruining performance.
@@ -75,10 +78,6 @@ public static void sort(int[] array) {
      * @param depth    the remaining depth for allowing concurrent execution
      */
     private static void concurrentMergeSort(int[] array, int[] temp, int left, int right, ThreadPoolExecutor executor, int depth) {
-        if (left >= right) {
-            return;
-        }
-
         int length = right - left + 1;
 
         // Switch to sequential sort if the array is small or we have reached the maximum concurrent depth

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

@@ -83,4 +83,28 @@ public void testSingleElementArray() {
 
         assertArrayEquals(expected, array, "Single element array should be handled without errors.");
     }
+
+    @Test
+    public void testNullArray() {
+        int[] array = null;
+        ConcurrentMergeSort.sort(array);
+        org.junit.jupiter.api.Assertions.assertNull(array, "Null array should be handled without errors.");
+    }
+
+    @Test
+    public void testInterruptedException() {
+        int[] array = new int[20000]; // Large enough to trigger concurrent threads
+        Thread.currentThread().interrupt();
+        try {
+            ConcurrentMergeSort.sort(array);
+        } catch (RuntimeException e) {
+            org.junit.jupiter.api.Assertions.assertTrue(
+                e.getCause() instanceof InterruptedException || e.getCause() instanceof java.util.concurrent.ExecutionException,
+                "Should catch and wrap InterruptedException or ExecutionException"
+            );
+        } finally {
+            // Clear interrupted status so it doesn't affect subsequent tests
+            Thread.interrupted();
+        }
+    }
 }