Return null from transformer when not transforming

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.

closesn #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;
         }
     }