[feature request] Do not print NA in filenames if metadata does not exist for it.

[rieje]

I use an output format suitable for Youtube videos, but for other sites, they don't use some of the metadata (or at least properly). In the videos from these sites, I often get several NA in the filename (e.g. Cat video - NA (NA).mp4).

How about an option to not print the metadata at all if it would report NA otherwise?

[dstftw]

What's the point? How come Cat video - ().mp4 is better?