style: include UTAO_JUNIT_ASSERTION_ODDITIES_USE_ASSERT_NULL (#7225)

spotbugs-exclude.xml

@@ -222,9 +222,6 @@
     <Match>
         <Bug pattern="UTAO_JUNIT_ASSERTION_ODDITIES_USE_ASSERT_NOT_NULL" />
     </Match>
-    <Match>
-        <Bug pattern="UTAO_JUNIT_ASSERTION_ODDITIES_USE_ASSERT_NULL" />
-    </Match>
     <Match>
         <Bug pattern="UTAO_JUNIT_ASSERTION_ODDITIES_IMPOSSIBLE_NULL" />
     </Match>

src/test/java/com/thealgorithms/ciphers/PermutationCipherTest.java

@@ -1,6 +1,7 @@
 package com.thealgorithms.ciphers;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import org.junit.jupiter.api.Test;
@@ -121,8 +122,8 @@ class PermutationCipherTest {
         String decrypted = cipher.decrypt(encrypted, key);
 
         // then
-        assertEquals(null, encrypted);
-        assertEquals(null, decrypted);
+        assertNull(encrypted);
+        assertNull(decrypted);
     }
 
     @Test

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

@@ -2,6 +2,7 @@ package com.thealgorithms.datastructures.trees;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNull;
 
 import org.junit.jupiter.api.Test;
 
@@ -30,7 +31,7 @@ public class TreapTest {
         treap.insert(3);
         treap.insert(8);
         treap.insert(1);
-        assertEquals(null, treap.search(4));
+        assertNull(treap.search(4));
     }
 
     @Test

src/test/java/com/thealgorithms/dynamicprogramming/LongestCommonSubsequenceTest.java

@@ -1,6 +1,7 @@
 package com.thealgorithms.dynamicprogramming;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
 
 import org.junit.jupiter.api.Test;
 
@@ -55,27 +56,24 @@ public class LongestCommonSubsequenceTest {
     public void testLCSWithNullFirstString() {
         String str1 = null;
         String str2 = "XYZ";
-        String expected = null; // Should return null if first string is null
         String result = LongestCommonSubsequence.getLCS(str1, str2);
-        assertEquals(expected, result);
+        assertNull(result);
     }
 
     @Test
     public void testLCSWithNullSecondString() {
         String str1 = "ABC";
         String str2 = null;
-        String expected = null; // Should return null if second string is null
         String result = LongestCommonSubsequence.getLCS(str1, str2);
-        assertEquals(expected, result);
+        assertNull(result);
     }
 
     @Test
     public void testLCSWithNullBothStrings() {
         String str1 = null;
         String str2 = null;
-        String expected = null; // Should return null if both strings are null
         String result = LongestCommonSubsequence.getLCS(str1, str2);
-        assertEquals(expected, result);
+        assertNull(result);
     }
 
     @Test