Add tests for NodeStack (#6009)

src/test/java/com/thealgorithms/datastructures/stacks/NodeStackTest.java

@@ -70,16 +70,4 @@ class NodeStackTest {
         stack.pop();
         assertEquals(0, stack.size(), "Size should be 0 after popping all elements.");
     }
-
-    @Test
-    void testPrint() {
-        NodeStack<Integer> stack = new NodeStack<>();
-        stack.push(1);
-        stack.push(2);
-        stack.push(3);
-
-        // Output verification would ideally be handled through a different means
-        // but you can print as a basic check to confirm method runs without errors.
-        stack.print();
-    }
 }

src/test/java/com/thealgorithms/datastructures/trees/BinaryTreeTest.java

@@ -1,6 +1,7 @@
 package com.thealgorithms.datastructures.trees;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.fail;
 
 import org.junit.jupiter.api.Test;
 
@@ -35,7 +36,12 @@ public class BinaryTreeTest {
         t.remove(5);
         t.remove(7);
 
-        assertEquals(t.getRoot().data, 9);
+        // Checks whether the root is null before accessing date
+        if (t.getRoot() != null) {
+            assertEquals(t.getRoot().data, 9);
+        } else {
+            fail("The root node is null after removal.");
+        }
     }
 
     // checks that removing an unexistend node returns false