[JENKINS-63828] It is legal to have a CloudProvisioningListener not implementing canProvision

I got confused in
https://github.com/jenkinsci/jenkins/commit/3992a30efb1abf697cae8e02f9e81dd04b92b135
by Cloud#canProvision which was abstract vs.
CloudProvisioningListener#canProvision which wasn't and was returning
null.

core/src/main/java/hudson/Util.java

@@ -1480,7 +1480,7 @@ public class Util {
         if (isOverridden(base, derived, methodName, types)) {
             return supplier.get();
         } else {
-            throw new AbstractMethodError("You must override at least one of the "
+            throw new AbstractMethodError("The class " + derived.getName() + " must override at least one of the "
                     + base.getSimpleName() + "." + methodName + " methods");
         }
     }

core/src/main/java/hudson/slaves/CloudProvisioningListener.java

@@ -42,13 +42,16 @@ public abstract class CloudProvisioningListener implements ExtensionPoint {
      */
     @Deprecated
     public CauseOfBlockage canProvision(Cloud cloud, Label label, int numExecutors) {
-        return Util.ifOverridden(() -> canProvision(cloud, new CloudState(label, 0), numExecutors),
-                CloudProvisioningListener.class,
+        if (Util.isOverridden(CloudProvisioningListener.class,
                 getClass(),
                 "canProvision",
                 Cloud.class,
                 CloudState.class,
-                int.class);
+                int.class)) {
+            return canProvision(cloud, new CloudState(label, 0), numExecutors);
+        } else {
+            return null;
+        }
     }
 
     /**