Wallabag: fix archive_finished initialization logic

[Frenzie]

Closes #14214.

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This change is 

[Frenzie]

Well, that's odd. I haven't known luacheck to fail at parsing basic Lua like this. Oh well, I'm not attached to doing it as a one-liner so I'll just placate it.

> bla = lala == nil and '1' or '2'
> print(bla)
1
> lala = ''
> bla = lala == nil and '1' or '2'
> print(bla)
2