Invalidate instead of evaluating empty script in scripted components

As of NIFI-8625, script engines that aren't Invocable are filtered out from the list of available engines to choose for most scripting components (except for ExecuteScript and ScriptedTransformRecord at the time of this writing) as all other scripting components require an Invocable script engine. However this causes ECMAScript to be the default engine, and it throws a NullPointerException when the script body is empty (which it is when the processor is first created).

Rather than make a different engine be the default (which could be done under a different Jira), it is better to not try to create the ScriptRunner and/or evaluate the script if the body is empty.