Allows overriding a key even if it has a default.

Not sure if this is going to have any side effects but I wanted to get it up for discussion.  Basically I have a master template for what "output" (future publishes) should be named.  I'd like to keep it all in one template so it's easier to manage in the long run both when configuring apps and making updates.

output_name: "{Entity}_{Task}_{name}[_{channel}][_{publish_type_short}][.{view}][.{SEQ}][.{extension}]"

I would like the SEQ template key to be optional but there doesn't seem to be a way to do that without the change I've included.  This change allows SEQ to be ignored if it is added to the fields passed to apply_fields and equals None.

Hopefully this makes some sense!

-tony