Prevent further processing and watchdog msg if URL is empty for whatever reason

If the $page_url in feeds_imagegrabber_feeds_set_target($node, $target, $page_url) is empty, FIG still goes through the process of validating etc, eventually firing a watchdog message "Input to field was not a valid URL".

Would it be ok to bail out earlier?

The reason this comes up for me is a very particular use case, but logically I think it still makes sense.

In my case many of the feed items will have a default image if nothing better is available. If I'd just let feeds and FIG do its thing, I would end up with thousands of copies of the same image. Hence I check for this default image in hook_feeds_after_parse() and replace the default image with an empty string. Since imagefield gives the option to have its own default image, this works well.

But I get a lot of unnecessary watchdog writes from this and FIG does more work than it needs to. Mini patch follows...