Fix #508

pf4j/src/main/java/org/pf4j/processor/ExtensionAnnotationProcessor.java

@@ -131,7 +131,9 @@ public class ExtensionAnnotationProcessor extends AbstractProcessor {
         }
 
         // write extensions
+        if (extensions.size() > 0) {
             storage.write(extensions);
+        }
 
         return false;
     }