fix: add newline at end of files for Checkstyle

Author: Ayush41

src/main/java/com/thealgorithms/recursion/Factorial.java

@@ -31,3 +31,4 @@ public static long factorial(int n) {
     return n * factorial(n - 1);
   }
 }
+

src/test/java/com/thealgorithms/recursion/FactorialTest.java

@@ -33,3 +33,4 @@ public void testNegativeNumberThrowsException() {
     assertThrows(IllegalArgumentException.class, () -> Factorial.factorial(-1));
   }
 }
+