Originally you said you either got a message that was empty or a message that could be processed. Therefore only messages that can be processed will have aaa in them. Who cares about the double quotes. You don’t need them.
I could tell you how to do it through the UI (just include the quotes, you need do nothing else). I don’t support file based configs any more. I’d rather spend my time addressing home automation problems, not syntax problems.
But in this case, you don’t need the quotes to be a part of the expression. In fact, if the only unparsable message is empty, (.+.*) will work. Empty messages will be dropped. Any message with one or more character will be passed to the JSONPATH. Since all non-empty messages are parsable by the JSONPATH that’s all you need.