#404 empty pngwalk, instead use existing product.vap

[Jeremy Faden]

Craig suggests that an empty autoplot should run using the product.vap file in the output pngwalk directory. We should at least look at the vap to see that something is done before clobbering the product.vap in the output directory.

Maybe if the overwrite is on, then go ahead. If it is not on, then we will only run if the current state is consistent with the vap.