Return empty/blank for missing metadatga instead of "NA".

[Snake883]

Checklist

• I'm reporting a feature request
• I've verified that I'm running youtube-dl version 2021.01.16
• I've searched the bugtracker for similar feature requests including closed ones

Description

For missing metadata in the Output Template, I would like an option for blank/empty instead of "NA".

For example, I want the playlist metadata if exists. But if playlist information does not exist, return empty.