Can nulled/downloaded themes/plugins affect AdSense?

Suppose you download a WP theme and use it and you apply for adsense.

Can that theme already have an adsense tied with and then google say you already have an adsense account when you yourself apply for?

What will be the get around? Because I could not find any adsense code in the header when I used that theme.

yes you can use nulled themes without any issue.

Nulled plugin and themes don't have any impact on AdSense (advising with experience).
Security Issue with Nulled plugin and themes itself is another point of discussion. Your nulled theme may have a backdoor or malicious script, and the owner can ruin your website's reputation overnight, steal your user's data or use your website to steal their data.

The decision is yours.

Can that theme already have an AdSense tied with and then google say you already have an AdSense account when you yourself apply for?

Yes, that might be possible that nulled theme creator has added Adsense code already in the theme.