style: include UTAO_JUNIT_ASSERTION_ODDITIES_USE_ASSERT_NULL (#7225)

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