[videodb] don't set the playcount on scan if missing episode/season numbers

When adding new episodes on scan we lookup to see if we have a previously matching episode/season for this show with an existing playcount in order (I'm guessing?) to preserve the playcount for duplicates.

This causes issues if the episode/season numbers are not specified, as any previous episode matches.

I'm guessing that we pretty much don't support very well the latter case (i.e. where aired date is specified but season/episode aren't).  Also, I don't really see the point of this codeblock - it only matches existing episodes that match season/episode number but are duplicates.  Seems to me that we should just be detecting the dupes instead?

Fixes #15234.

@vdrfan, @Montellese please take a look.