Return null from transformer when not transforming (elastic#125961) (elastic#125973)

The transform API for instrumentation should return null when no
transformation occurs. This commit fixes our entitlement transformer to
return null instead of the original buffer.

closes elastic#125920

Author: rjernst

libs/entitlement/src/main/java/org/elasticsearch/entitlement/instrumentation/Transformer.java

@@ -46,7 +46,7 @@ public byte[] transform(
             return instrumenter.instrumentClass(className, classfileBuffer, verifyClasses);
         } else {
             // System.out.println("Not transforming " + className);
-            return classfileBuffer;
+            return null;
         }
     }