raindrops: standardize starter implementation

exercises/raindrops/src/example/java/RaindropConverter.java

-public class RaindropConverter {
+class RaindropConverter {
 
-    public String convert(int number) {
+    String convert(int number) {
         String result = "";
 
         if (number % 3 == 0) {
             result += "Plong";
         }
 
-        if (result == null || result.isEmpty()) {
+        if (result.isEmpty()) {
             result = Integer.toString(number);
         }
 
         return result;
     }
+
 }

exercises/raindrops/src/main/java/RaindropConverter.java

+class RaindropConverter {
+
+    String convert(int number) {
+        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
+    }
+
+}