Empty value causes error on "String to Unix timestamp"

I was able to sort this issue by using a rewrite plugin on top of "String to Unix timestamp." I used a token on the rewrite so I can use another field as default value. You can also use a "Set value or default value" https://www.drupal.org/project/feeds_tamper/issues/3029164 →
Them I set a conditional on the template so it does not render the default value.

Ticket needs update

Adding related issue. I feel like this probably could also be moved to the tamper issue queue if this is not specific to feeds tamper.

Thank you for sharing your work around analaura!

This fix from Tamper module:
https://www.drupal.org/project/tamper/issues/3119301 ✨ Skip tamper if item is empty Fixed
might be good solution. It provides new tamper "Skip tampers on empty". I've added this tamper to every field that I'm trying to manipulate and it works fine