Add input validation for negative values in FibonacciSeries (#7177)

Author: aadislnk

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

@@ -12,10 +12,12 @@ private FibonacciSeries() {
         throw new UnsupportedOperationException("Utility class");
     }
     public static int fibonacci(int n) {
+        if (n < 0) {
+            throw new IllegalArgumentException("n must be a non-negative integer");
+        }
         if (n <= 1) {
             return n;
-        } else {
-            return fibonacci(n - 1) + fibonacci(n - 2);
         }
+        return fibonacci(n - 1) + fibonacci(n - 2);
     }
 }