Skip extensions with null values

Author: damianmalczewski

src/main/java/io/github/malczuuu/problem4j/core/ProblemBuilderImpl.java

@@ -66,7 +66,7 @@ public ProblemBuilder instance(String instance) {
 
   @Override
   public ProblemBuilder extension(String name, Object value) {
-    if (name != null) {
+    if (name != null && value != null) {
       extensions.put(name, value);
     }
     return this;